There are several CAD/CAM programs out there for MacOS. Here is a list of a few: http://www.macuser.de/forum/f53/3d-2d-cad-395058/#post4499316
I've also read good things about punchCAD: http://www.punchcad.com/
Of course, there is always SketchUp if you are in a pinch. There are a few CAM plugins for SketchUp as well, but I've never used them, so I can't vouch for them.
CAD/CAM will get you as far as designing your parts and producing the g-code, but if you also want to run the CNC machine with your Mac, you will need a piece of software similar to Mach3 or EMC to run the g-code. I haven't been able to find anything like these programs that runs on Mac, so you may have to run Mach3 or EMC in Windows using Boot Camp or a virtual machine such as VMware or Parallels.
Personally, I would use the Mac to do all the CAD/CAM, then use a cheap PC to run the CNC machine.
EasyDNC for OSX
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DOES MACH3 WORK ON A MAC?