What makes Musescore create a mixer for an instrument

I'm fiddling around with some sound fonts, and I got the samples and sound font to be loaded in and the drum set files even work and show up. but when I click on my instrument it doesn't make a mixer it just makes the notation. If I put my sound font on a premade instrument it works fine. So, I was wondering is the code to make it make a mixer instance for it in the instrument.xml file or in the sound font file, or somewhere else. Thanks