Originally Posted by Mike Halloran
Did you click OK/Yes?
Yes.
Originally Posted by Andrew - PG Music
There's the 'individual' SongPicker, which you can open by shift-clicking the SongPicker button (the button that has an image of two eighth notes, to the left of the song title), and there's the SongPicker that's part of the 'MultiPicker' dialog, which you can open by ctrl-clicking the button. By default a click without a modifier key will open the MultiPicker, unless you've changed the default.
I tried shift-clicking and it opened! I even tried closing and reopening to see if it was a fluke, but the SongPicker opened. Now I can click without the shift key and it still opens. I don't know how that fixed anything, but now it works!
Originally Posted by Andrew - PG Music
-It may be that there is some kind of error trying to open the folder you have set as the home folder (default is /Applications/Band-in-a-Box/MySongs).
I never changed any default locations, but if this happens again I'll try some of the other things you recommended.

I have no clue what ultimately fixed the problem, but thank you to all who chimed in to help!