You can run my G-Wizard G-Code Editor on the Mac as it is written in Flash. Lots of Mac and Linux users too for the calculator, so I know it works well there. Works on Windows too, of course.
It's free while in beta, but that won't be forever. Meanwhile, enjoy!
You can register for Beta and download here:
GWizardE: A Machinist's CNC G-Code Editor
Not sure I've ever seen another sim that runs on Mac. I'll be curious to see if you get any other responses.
Cheers,
Bob Warfield
Try G-Wizard Machinist's Calculator for free:
http://www.cnccookbook.com/CCGWizard.html