You know google can help sometimes.
After spending a whole 3 minutes on these topics I found that first, MINIUSF is an N64 audio format.
A few searches later and I found this... USF WinAmpPlugin
Just extract and place the dll in your sound directory.
Then add this line to your mugen.cfg under the music plugin section:
plugin = plugins/in_usf.dll, usf, miniusf
Hope that helps.