Hi,
I think you can do this with Finale's free Notepad editor. Just open up the .mus file (if it is backwards compatible) and then save as a .mid file.
You can download their Notepad (it's free) by registering at Finale's website (
www.finalemusic.com) You'll need to go to their Download page, search for the "Notepad" product, and then you have to register with them before you can download it.
ttfn,
Sherry