I have answered in several threads with the same question, so please use the search function before posting next time.

Anyway, what you need to do is got to preferences->sharing in LW and check which folder is your 'shared' folder. Navigate to that folder and put all the music that you want in your iTunes library inside a new folder. Now open iTunes and select file->add to library. Now choose the folder with all your mp3's and you're good to go.