At the very top of this page there is a "Support" link.
If you hit that and then hit "FAQ" (Frequently Asked Questions) link, scroll down to BB ver 12 for Macintosh, you can find the answers to a whole lot of questions, including this one.
Use page search feature (typically ctrl-f in most browsers) to quickly search the page for keywords.
So I entered "Transpose" and it immediately returned BIAB FAQ #36:
Quote:
36. How do I set the right key or visually transpose the music for my non-concert instrument?
The key transpose box on the main screen of Band-in-a-Box sets MIDI music you hear and the Notation key for viewing and printing at the same time. The 'Transpose' setting in the Notation Window Options dialog affects only the viewable and printable notes, while leaving the MIDI playback in the original key. This allows you to view proper notation for non-concert instruments, such as a tenor sax. Open the Notation Window and press the [OPT] button to open the Notation Window Options dialog. Beside 'Transpose', type in the number of semitones that you want the notation to be transposed. For example, if you have a trumpet, set it to +2.
So it is just like earlier Windows versions, from the Notation Window view, hit the Options button and look for the transpose shift stuff. Should be a setting there labeled for Bb Trumpet. If not that fancy, what you want is a whole step up from Concert C, which is two half steps up, or that +2 entry.
--Mac