Hello Thanasis,
As far as I can say/remember (!) microtones in ML2 are limited to 1/4 or 1/8th tones and work (as in OM) by detuning MIDI channels >= 2 according to the selected tone subdivision.
So in principle you should just select an active MIDI device and ML2 will send notes (eventually transposed via MIDI pitchbend) to it.
There is no specific connection in ML2 with MicroPlayer for microtone playback via OSC (contrary to OM) but Microplayer can also be used as a “normal” MIDI device, and you can see his bm-microton MIDI in/out port in your interface. I am not sure, however, if bm-microton as a MIDI device will correctly handle pitchbend events, and let you hear microtones.
So my advice is just to use any regular MIDI synthesizer (e.g. SimpleSynth: http://notahat.com/simplesynth/)
ps. Please note: Musique Lab 2 is no longer maintained and supported on recent versions of macOS.
Hope this helps.
—J