If you really need to split your MIDI channels in Band-in-a-Box, you can work around this by importing the MIDI file into the Melody Track, then going Edit > Copy Special > Copy/ Move Tracks. Here you can select the individual channels and move them to another track in the Mixer.

thank you yes need to do it.

But is there a tutorial? Thats very confusing. One, there
is no trombone choice in the pull down menu.

I don't see how that complicated box separates tracks.