Its really not all that complex.
First, do you even know if the MIDI is installed on only the right, left or both hands? Next, find out what channels the MIDI uses on the treble, bass and chords. If all 3 are in use the most common setup is CH1 treble, CH2 bass and CH3 chords, but there is no guarantee yours is configured in that way.
The accordion is basically a MIDI out, meaning it sends signals to a Digital instrument that has a MIDI cable going from the accordion (MIDI out) to it's MIDI in socket. If for the xample your accordion keyboard is set to CH1, you configure a specific part of the MIDI instrument to receive on CH1. When you play the accordion treble, digital music comes out of the digital instrument based on what notes you touch on your accordion.
I am not going to go further until I know what your goals are... To set it up with a MIDI keyboard or an arranger. The process is similar but I am not going to start to explain the process for both if I do not have to.
What are your goals with the accordion and what MIDI equipment do you have or what do you want to do?
BTW, you can also go to YouTube and type in "MIDI basics" or similar for more info.