loading songs to Clip from a MAC

I had all the Clip freezing issues and fixed them by reformatting. Now I just need to get song on it to play. When I drag and drop from itunes, it copies fine, but when I try to play the song, it says empty. I did notice the file extention is m4u not mp3. If this is the problem, how do I convert the files to mp3.

Any suggestion?