Hello,
As a fellow Mac user, I sympathize with your plight. Here's what I've found:
1) Manhattan GMAT's online content is awesome and, because it's web-based, can be used by any computer type/platform/operating system. Go to
https://www.manhattangmat.com and then go to their MGMAT Store and click on the computer-based products. They've got loads of practice questions (eight question banks in all), on top of six full-length computer-adaptive practice exams, and, an extra question bank of very difficult Quant problems. Very useful, very efficient. Can be used anywhere there's a wireless network. The cost for the whole computer bundle (6 CAT exams, 8 questions banks, diagnostic math section, and challenge problem bank is $70 and well worth it).
2) Grockit's "game" (read: practice) system is also web-based and thus, compatible with everything. Go to
https://grockit.com/ to learn more. Again, anywhere you've got a wireless network, you can log in. The games are also computer responsive and interactive, so you can practice online with fellow students. Very cool setup.
3) Princeton Review and Kaplan both have online content as well, and I think you can register for free online tests if you want to try them out. The registration process is fairly harmless and, if you like what you see, you can buy more online stuff from them, too.
Good luck!
Michal