Inability to chose specific sounds from Musescore Basic & SOundfonts

The problem is that when you go to your mixer and change the sound of your instrument, you can chose any instrument from Musesounds, but there is no way to chose which sound you use from Musescore Basic or any sound fonts you download. When you click on a sound font or Musescore Basic, it just automatically choses the instrument that the line is labeled as. This is a problem because, for example, I downloaded the Pokémon Black and White soundfont, but I can't use all the instruments in there because I have no way of selecting them. I can use the sound Musescore assigns to each instrument, but nothing else.

-click "Mixer"
click "sound" for an instrument
Go to "soundfonts"
Notice that there is no dropdown menu letting you chose which instrument sound you want to use