Why can't Band-in-a-Box files with file extensions .mgu, .mgx, .sgu, .mg1... not being used with the Band-in-a-Box Plugin?
.mgu, .mgx, .sgu, .mg1... are not style files. Only files with the extension .sty are styles files and can be used with the Band-in-a-Box Plugin for notation composer.
On the PG Music website these files are called "MIDI Styles Sets for Band-in-a-Box®"