Howdy,
You can use the Staff/Split melody and accompaniment feature. This takes the top (typically melody notes) and puts them in a separate staff from the remaining "accompaniment" staff. If you don't like how Musician has split the notes (or if it has made a mistake because of off-beat notes), then you can select the note and use the "Shift + arrow key" to shift the note to the staff above (up arrow key) or below (down arrow key) it.
Enjoy
Sherry