On ProjectPlaylist, after you search for the song. Click Visit Site next to the song name. Make sure the song works though by previewing it. Then in the top left hand corner under Search Result Location: will be a URL. Copy and paste that URL of the MP3 into the music player on your profile.

Mission said this: To add music, first you need to find some music (try this link: Web 2.0 Mashup: Best of the Internet Today) Then click the wrench/edit icon to the right of XPSF Music Player and put in the link for the song under the section for track #1 (plus any additional fields you want to fill in.) Then save it.

But I personally went to http://www.projectplaylist.com to find some URL's for the MP3's.