help please :)

How about opening a new file with a grand staff (have both files open) and then copying (use edit->select all or ctrl+a) the imported-midi notes and pasting them into the fresh grand-staffed score? MuseScore uses tabs for multiple scores.
You can also hold onto shift+control and press up/down arrows to move a chord/note to one staff above/below. Hope this helps. Good luck with your assignment :)

Welcome aboard...
Did you create a midi file of your melody line, and then open the midi in MuseScore?
Midi is more of a 'note event' recorder - generating machine instructions for music playback - rather than a tool with a focus on music notation. Converting midi into musical notation has its problems and shortcomings, as a search through the forum topics will attest.

So...
Why not start off with a blank grand staff within MuseScore and then enter your notes directly.
See:
https://musescore.org/en/handbook/note-input

Regards.

If you post the specific file you are having trouble with, we can usually offer better help. Normally, if the MIDI file was created specific using a piano sound, it should import as two staves automatically. So presumably this MIDI file wasn't created that way. But you should be able to force it to display as a grand staff by checking ghe "Split staff" option in the import panel that displays at the bottom of the screen when you import a MIDI file.