Steve,
I just walked thru this, and it did not require anything special.
1) Plugin SD Card reader to Mac
2) Insert SD Card, wait for Mac to recognize
3) Copy folder of MP3s from Mac to top level of SD Card - Drag folder of music and drop on to SD Card icon in Finder. No sub-folders in the folder, just mp3s.
4) Eject SD Card
5) Place SD Card into BS2
6) Press and hold both down arrows until it beeps. It beeps every 3 seconds, then starts playing. New music is recognized.
Sorry, but it didn't require any magic, which makes me think there might be a problem with your BS2... 🙁
Stan