I tried WAV and it didn't work either. Searching more deeply, I found another thread on the same subject. It seems that the method given in the help file for importing from iTunes to GarageBand on iPad is actually not implemented. See https://discussions.apple.com/message/13228478#13228478 Some workarounds can be found there. The easiest thing seems to be to just record analog using either an iRig or just play the song from your computer speakers into the iPad mic. Of course, that's pretty slow, but for making a short loop it's ok.
you can email the file to yourself, and use an app called hokusai to open it. From there you can use audiocopy and then paste the song as a sample into a garageband track. My tutorial will give you some clues.