It always bugged me there wasn't an easy way to relay 1 message in the PLC script.   I came up with this idea about 2 weeks ago.   It works pretty good for what I'm doing.   Hopefully it can help others out as well.  

I was inspired by needing to find a way to run a function in the PLC script once enable was pressed the very first time.   Did you know there is a register that keeps track of how many times you enable/disable Mach?   Pretty cool.  
I used the first instance but it kept looping.   So I changed the register to 2 once I was finished running the function, that solved the looping.
This clicked for messages. Use Registers... VARS#

Check page 1 for more info

Craig,
Here is a test I have set up on my computer at work.
I have a button to toggle Input10 on and off.  I added a message to it.  It runs in the PLC Script and only logs 1 message per button press.

     local inst = mc.mcGetInstance()
     local MessageVar = mc.mcCntlGetPoundVar(inst, 400)
     local Input10 = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT10)
     local hSig = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT10)
     local State = mc.mcSignalGetState(hSig)
     if State == 1 then
         scr.SetProperty("TestTextBox", "Value", "On")
         if MessageVar == 0 then
             mc.mcCntlSetLastError(inst, "Input 10 is on.")
             mc.mcCntlSetPoundVar(inst, 400, 1)
         end
     else
         scr.SetProperty("TestTextBox", "Value", "Off")
         mc.mcCntlSetPoundVar(inst, 400, 0)
     end

you guys went way over my head so i programmed it into the screen load and plc script with a few modifications

screen load
--2CNC433--Mach4 4381
pageId = 0
screenId = 0
testcount = 0
machState = 0
machStateOld = -1
machEnabled = 0
machWasEnabled = 0
inst = mc.mcGetInstance()

---------------------------------------------------------------
-- Signal Library
---------------------------------------------------------------
SigLib = {
[mc.OSIG_MACHINE_ENABLED] = function (state)
    machEnabled = state;
    ButtonEnable()
end,

[mc.ISIG_INPUT0] = function (state)
    if (state == 1) then
        wx.wxMessageBox('Way Oiler Empty Add Oil')
    end
end,

[mc.ISIG_INPUT1] = function (state)
    if (state == 1) then   
        CycleStart()
    end
end,
[mc.ISIG_INPUT2] = function (state)
    if (state == 1) then
        mc.mcCntlFeedHold (0)
    end
end,
[mc.ISIG_INPUT3] = function (state)
    if (state == 1) then   
        CycleStop()
    end
end,
[mc.ISIG_INPUT4] = function (state)
    if (state == 1) then   
        mc.mcCntlEStop(0)
        wx.wxMessageBox('Vfd Fault')   
    end
end,
[mc.ISIG_INPUT5] = function (state)
    if (state == 1) then   
       mc.mcCntlEStop(0)
       wx.wxMessageBox('Drive Fault')
    end
end,
[mc.ISIG_INPUT9] = function (state)
    if (state == 1) then   
        AxisSelect()
    end
end,
[mc.ISIG_INPUT10] = function (state)
    if (state == 1) then   
        AxisSelect()
    end
end,
[mc.ISIG_INPUT11] = function (state)
    if (state == 1) then   
        AxisSelect()
    end
end,
[mc.ISIG_INPUT12] = function (state)
    if (state == 1) then   
        AxisSelect()
    end
end,
[mc.ISIG_INPUT13] = function (state)
    if (state == 1) then   
        AxisSelect()
    end
end,
[mc.ISIG_INPUT14] = function (state)
    if (state == 1) then   
        IncSelect()
    end
end,
[mc.ISIG_INPUT15] = function (state)
    if (state == 1) then   
        IncSelect()
    end
end,
[mc.ISIG_INPUT16] = function (state)
    if (state == 1) then   
        IncSelect()
    end
end,


and this is my plc modifications

--2CNC433--Mach4 4381
local inst = mc.mcGetInstance()
local rc = 0;
testcount = testcount + 1
machState, rc = mc.mcCntlGetState(inst);
local inCycle = mc.mcCntlIsInCycle(inst);

-------------------------------------------------------
--  Coroutine resume
-------------------------------------------------------
if (wait ~= nil) and (machState == 0) then --wait exist and state == idle
   local state = coroutine.status(wait)
    if state == "suspended" then --wait is suspended
        coroutine.resume(wait)
    end
end
---------------------------------------------------------------
--Axis Selection
---------------------------------------------------------------
function AxisSelect()
    local hsigInput9, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT9)
    local NoneSelected, rc = mc.mcSignalGetState(hsigInput9)
    local hsigInput10, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT10)
    local Xselection, rc = mc.mcSignalGetState(hsigInput10)
    local hsigInput11, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT11)
    local Yselection, rc = mc.mcSignalGetState(hsigInput11)
    local hsigInput12, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT12)
    local Zselection, rc = mc.mcSignalGetState(hsigInput12)
    local hsigInput13, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT13)
    local Aselection, rc = mc.mcSignalGetState(hsigInput13)
    if NoneSelected == 1 then
        mc.mcMpgSetAxis(inst, 0, -1)
        --mc.mcCntlSetLastError(inst, "No Axis Selected")
    elseif Xselection == 1 then
        mc.mcMpgSetAxis(inst, 0, 0)
        --mc.mcCntlSetLastError(inst, "X Selected")
    elseif Yselection == 1 then
        mc.mcMpgSetAxis(inst, 0, 1)
        --mc.mcCntlSetLastError(inst, "Y Selected")
    elseif Zselection == 1 then
        mc.mcMpgSetAxis(inst, 0, 2)
        --mc.mcCntlSetLastError(inst, "Z Selected")
    elseif Aselection == 1 then
        mc.mcMpgSetAxis(inst, 0, 3)
        --mc.mcCntlSetLastError(inst, "A Selected")
    end
end
---------------------------------------------------------------
--Increment Selection
---------------------------------------------------------------
function IncSelect()
    local hsig, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT14)
    local Step01, rc = mc.mcSignalGetState(hsig) -- increment .01
    local hsig, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT15)
    local Step001, rc = mc.mcSignalGetState(hsig) -- increment .001
    local hsig, rc = mc.mcSignalGetHandle(inst, mc.ISIG_INPUT16)
    local Step0001, rc = mc.mcSignalGetState(hsig) -- increment .0001
    if Step01 == 1 then
        mc.mcMpgSetInc(inst, 0, .010)
        --mc.mcCntlSetLastError(inst, "X1 Selected")
    elseif Step001 == 1 then
        mc.mcMpgSetInc(inst, 0, .001)
        --mc.mcCntlSetLastError(inst, "X10 Selected")
    elseif Step0001 == 1 then
        mc.mcMpgSetInc(inst, 0, .0001)
        --mc.mcCntlSetLastError(inst, "X100 Selected")
    end
end
-------------------------------------------------------
--Run Lube
-------------------------------------------------------
local inst = mc.mcGetInstance()
local hsig,rc = mc.mcSignalGetHandle(inst, mc.OSIG_OUTPUT0)
if (machState == 0)then
    mc.mcSignalSetState(hsig,0)
else
    mc.mcSignalSetState(hsig,1)
end

now i get this error
signal script error
lua:error while running chunk
[string ""]:75:attempt to call global 'AxisSelect' (a nil function)
stack traceback
[string ""]:75: in function '?'
[string ""]:706: in function <[string ""] 703>
correct the error and restart

i also started a help ticket but here can help more people
as i said this worked fine i switched and switched and played with it. it was faster and more responsive.
then i boot up today and it gives me this error which i dont have the foggiest idea how to find much less solve.
thank you
mark

Functions go in the screen load script.  You have your functions in the PLC Script.

thank you that was the boost i needed it works fine now.
mark