Here is the pinout for the 37 key model from the Doepfer website:
The 44 key has the same pinout on the connector with the addition of another lot of MK/BR contacts on the last two unused pins. The order goes round clockwise so the far right pin is MK5 and then other BR5.
The keyboard pins will be wired up to digital pins on the Arduino so they can be scanned to detect keypresses. At this stage velocity sensitivity is not required so the timing data between break and make is not required, which makes the programming a lot easier.