By default, Box Of Stops comes with standard set of instruments defined in the Midi standard - see General Midi 1.
Pretty much every keyboard comes with all these instruments, otherwise it doesn't qualify as GM compatible. Even if they aren't all apparent on the controls for your keyboard, they are probably available to Box Of Stops to select.
However most keyboards also use a more specialist set of sounds, these are probably the instruments you can see on the keyboard controls. To take full advantage of these you may need to find or create an Instrument set for Box Of Stops. This is essentially a list of instrument names and their appropriate Midi instructions.