I don't know Logic, but I'm fairly familiar with Macs. They generally make things pretty easy. Are you sure it's an Mp3? If it's in iTunes, it's probably an AAC file. In any event, you probably have to convert it (to wave, I'm assuming). Look in your pull-down menus for "import file," "add file," "convert file," or some such, or look for that in your Help file. Also, you could try burning your original file to a CD, then re-ripping that file into Logic.
GJ