You also need to go into your FileExchange control panel and designate mp3's as iTunes files. You should also do this in your web browser preferences, as well (even though that has nothing to do with LimeWire.)
Also, go into RealPlayer and make sure it isn't set to make itself your default mp3 player. When RealPlayer is installed, it assumes ownership of darn near every type of media file possible. I hate that.