^^^^ Exactly
The Mini MK3 doesn't generate sounds, so nothing there to feed the mixer. Either run it's USB through your computer for sounds (DAW ?) and use an audio interface's monitor outs to feed the mixer, or step up to something like the MPK mini plus 37, which has MIDI IN/OUT that you can plug into the interface - which feeds USB to the computer (DAW) for sounds.. and again use the interface's monitor outs to feed the mixer.