There is no mp3 player in the Mac version of Limewire, since we have iTunes and any number of quite good mp3 players available.

What do the icons of your downloaded mp3's look like? QuickTime, Real, iTunes or generic? Either way, the easiest solution is to get a freeware program at versiontracker.com called "iTunes Associator". This will fix the creator/file type automatically so the files open in iTunes. You can also do this at your leisure in the File Exchange control panel, though sometimes LW overrides these prefs.

Note, however, that there are a few kinds of Windows-ripped mp3's that iTunes won't play, though Audion and some of the others might.