How do I add a midi file that I've generated from another program to BiaB for Mac? I'd like to add additional instruments to the song, which is basically piano-based.

File > Import > Import Chords, Tracks and Lyrics From MIDI File or

File > Import > Melody From MIDI File or

File > Import > Soloist From MIDI File.

See the explanations under the File section of Chapter 15: Reference of the Band-in-a-Box for Mac Users Manual.