I made a tool change program that would put the tool up when you call and T0M6. So Mach4 will read Tool 0. Just put a conditional if selectedtool == 0 then...
But you should also be sure that you pick up the next tool correctly. You don't want your carousel to move forward into the spindle expecting to grab a tool to put away... you need to have your spindle move up first (assuming it is an umbrella type tool changer). if currentool == o then.....
As far as calling your T0M6 from a macro, I'm not sure you can do that. You can try it; put an mc.mccntlmdiexecute("inst", CODE).
But I would probably just put the T0M6 on the block above the M30.