You can play Unreal in OS X natively if you have Unreal Tournament and use the OldSkool mod. Once that is installed, I put the entire Unreal folder into Unreal Tournament. I then added the appropriate file paths to the UnrealTournament.ini file and was able to play Unreal in OS X. I've posted more detailed instructions at various forums. I may or may not have posted them here a long time ago.
An old usenet post (the link to get the old skool mod no longer seems to work).
Usenet post
Also look for the single player Operation Na Pali mod as it includes OldSkool.