Mac/Windows compatible CDs if they predate 2006 won't work with Mac OS X 10.7 or later unless you run Windows on your Mac, or run 10.6 in virtualization because old PowerPC Mac software are only supported through Mac OS X 10.6.8 with the software in 10.6 called Rosetta. Some of that software may not work if there are very particular PowerPC drivers that need addressing.
The Full Sims 3 is available for Intel Macs, according to these system requirements. I would contact them to determine if they have tested it in 10.7 or later, as they may be trying to access older code from PowerPC Macs that wasn't completely eliminated in 10.5.7, the minimum system they say it will run on. Be sure to find your GPU in the System Profiler and see if it meets those specs. Maybe you just need to patch the Sims package.
P.S. Some Windows users and developers aren't aware that Mac is the nickname for Macintosh, and MAC is the acronym for Media Access Control the hexadecimal code identifying certain network devices.