How do you play team fortress 2 on a mac OS X

It's possible with a third party application, Codeweaver's "Crossover"

Its not free, £25.99 but I have used it with success in the past.

A free alternative might be "WINE" which is a Linux utility for running Windows applications of a Unix system. However I don't know if there is version of mac osx and it may not be compatible with Steam/TF2.

Your best bet, other than Boot Camp with XP, in my opinion would be Crossover. Good luck