Hi Bruce
Yes I remember this now - I'm not sure why Mach has a problem with G43 - afterall it's in the gcode list that Mach reckons it supports. It doesn't matter of course because by using CAM your toollength offsets are already taken into consideration.
Anyway here's the way I got rid of this particular problem with mine.
If you run the post editor, the problem is that it can be difficult to find where (under which tab) any particular gcode actually is, well it is for me anyway! so...
Go to your VM instalation folder (presumably under "c:\program files" or similar.
Look for the "posts" folder.
Find Mach3.spm or more likely the Mach2.spm and load it into wordpad or notepad.
Do a find for G43 - you'll find it first under the MISCELLANEOUS DEFINITION SECTION - ignore this.
Continue the find and you'll see it in the TOOLCHANGE DEFINITION SECTION - You can either edit it out there and then, or probably safer go back to the post editor and take the line out there (you now know to look under the ToolChange Tab for G43)
Save your post as Mach3 (or whatever) and away you go.
Personally I find it easier just to edit the .spm file but until you're comfortable with the layout you're probably better off doing it in the editor as no doubt corruption of the file will screw things up right royally.
Hope this helps
Ian