Congratulations, those are nice computers, I wish I had one myself but unfortunately can't afford one.
Regarding the pk3 files, I don't know off-hand what programs are available for macs that can make zip files, I'm afraid the only advice I can give is to try different compression settings until you can match the exact same format as the original pk3. I realize that probably isn't much help, but being only barely familiar with how to operate a mac and (unhappily) not owning one I'm not in a good position to offer more advice for you.
I know a guy I see once in a while who does own a mac though, and he actually multi-boots his machines with various types of linux and sometimes even windows, and does all kind of crazy, advanced stuff with his gear. He's a musician though, I don't know if he's into gaming at all but I'll see if I can get a reply from him at some point, although it may take a while...