The manual has an example of using a probe to find the centre and diameter of a hole (section 10.7.12.3).
It stores the value for the calculated average diameter in paramater #1034

How to I know what the value is, after the program has run?
....
N320 #1034=[[#1014 + #1024] / 2.0] (find average hole diameter)
...

I think you can do a GetVar(1034) in VB...

I was suggested to send the value to the message bar, if possible, how would this be done?
 

You could do a macro like this:

Test = GetVar(1034)
Code("Msg, The Dia = " & Test)

OR you coudl do this
 Test = GetVar(1034)
MsgBox("Msg, The Dia = " & Test)