You have three options for poker on the mac. First, and probably the best, is using a XP virtualization package like Parallels. You need to buy parallels, and then XP (if you don't have a license) but you are basically running XP within the Mac OS. Second of course is Boot Camp. Apple gives this one away and is a dual-boot with another operating system (like XP). These will work with just about any poker client (Poker Stars, FTP).

Your third option is a beta that Pokerstars has available. I dont know how feature complete it is, but its a native Mac application. Check with P* technical support.