The notation window is not as fully developed in Mixcraft as in dedicated score editors.
Bb minor is the enharmonic equivalent to A# minor. The key signature for A# minor has 7 sharps. Technically, if we call it Bb it should use flats instead of sharps - obviously! And Bb is normally used rather than A#.http://en.wikipedia.org/wiki/B-flat_minorhttp://en.wikipedia.org/wiki/A-sharp_minor
For transposing multi-part midi files, using "Adjust To Project Key" usually does the trick.