Bought a used Megadrum with inputs not labeled

[dmitri]

I guess the easiest way it to assign each input a unique note and then connect a trigger to each input and see what inputs are triggered.
What firmware version is it running?

[dmitri]

I can only test MDM with recent versions of Mac OS. Either upgrade your Mac OS or run Linux or Windows in as a VM (e.g. in VirtualBox) and run MDM from there. Did you identify inputs?