I just got the updated Garage Band app for iOS 6. One of the features is the ability to import songs from my music collection. Does anybody know how to get that to work? I have "the new iPad" and just updated to iOS 6.
Found your post by googling around looking for an answer myself. Once I found my answer, I figured I'd come share it.
So, it doesn't make a lot of sense, but:
Choose the "sampler" instrument, then tap the loop icon in the top right corner. From there, you can pick an apple-provided loop, files imported via iTunes file sharing, or songs from your music folder.
It's not really intuitive at all. I only found it by going through just about every option.