Cartierusm,
First off, nice job on the lathe! It looks great and I'm glad you got it working!
Second, Mach4 lathe doesn't read G90 or G91 like the Mill does. G90 is implied when using X or Z. To move incremental you use U for X and W for Z.
Example: G00 U3.00 W3.00 is the same as G91 G00 X3.00 Z3.00.
Third, Although I agree with Chaoticone to move to a tool change position in the code, I can see why you would want to put it in your M6 macro.
Like Chaoticone said, it will be easier to put it in a button but having it embedded in the M6 would be cool. You will indeed have to make a module to utilize a function in a macro. Daz has a video on modules that you can watch, its pretty comprehensive and I was able to make a module after watching his video.
Again. Great work on the tool changer!