I'm still not sure what you are trying to do, so I'll answer a bit generically.
The Keygrabber software translates from the button you are using into the virtual keystroke that you select to match it. Each time you press the button, the matched keystroke is sent to MACH. You can check your work by using the keyboard to send the same keystrokes. If it works from the keyboard, then the issue lies with either the setup or the capabilities of Keygrabber. If MACH still doesn't behave the way you expect when you are sending the keystrokes directly from the keyboard, then it's either a setup problem (maybe a typo in the screen setup on the MPG assigning the keystroke or some incompatibility with the specific character key you've chosen) or you are trying to make MACH do something that Art has not written in (it's always his fault anyway :-) ), or a confusion on your part about how the specific function is designed to work.
I know this isn't the specific answer, but it's the best I can do since I don't really understand what you are trying to accomplish. At least it gives you a way (hopefully) to troubleshoot the problem. If you can go into some detail of step-by-stem what you would like the tool to do and what specific steps you are taking to try this procedure, I think it may clear some of the fogs of confusion away for Brian, Art, myself and others.
Hope this helps a bit.
Jeff