Leave it as a PK3!!! Us Mac user wants this too!!!
Look here (
http://www.lucasforums.com/showthread.php?s=&threadid=131849), starting with the post by Brad Oliver, for all the information you need on this and a logical place to respond to the issue.
We'll have a pk3 version available for MAC users, but you will be on your own getting it to work, and some of the menu's and videos may not work, if the Mac game behaves the same as the PC version . . . .