How to connect AKSequencer to a AKCallbackInstrument?

typewriter found the solution to my problem. Here is the code which works now, printing the midi number of the note each time it is played (but I didn't add sound yet) :

// dont write the .mid extension in filename :
let sequencer = AKSequencer(filename:"coucou") 
let callbackInstr = AKMIDICallbackInstrument()
callbackInstr.callback = myCallBack
sequencer.setGlobalMIDIOutput(callbackInstr.midiIn)
sequencer.play()

func myCallBack(a:UInt8, b:MIDINoteNumber, c:MIDIVelocity) -> () {
    if (a == 144) {  // if noteOn
        print(b)
    }
}

True, the class AKCallbackInstrument does not have a property midiIn, although the documentation does show it being used that way. Instead of using AKCallbackInstrument, use AKMIDICallbackInstrument. That class has midiIn, and seems to work fine.