There are jumpers bottom-middle of the board. JP1 and JP2 needs to be open for PS3, You need to close JP2 for Wii, but JP1 stays open.
What I did is soldering a dip switch to my PS3 MPA and I can switch at will, should be the same for a Wii one, but I didn't test personally. Also, if you plan to use the MPA for a keyboard, I suggest soldering a jack input for a pedal to the Overdrive button while at it, assuming you care about overdrive activation, it's much better using a pedal than searching for that button on the MPA.
It was speculated that the same technique should work on instruments/dongles, assuming they have the jumpers. This won't work with the xbox devices as they require some sort of special chip or something.
Thanks a lot, this is very informative!
You just reminded me that I had found someone explaining the same kind of trick for the keyboard, when we bought the wrong one. Then eventually we just have returned it and bought the PS3 one from someone else, so I didn't try the trick myself.
So apparently the MPA is similar, it doesn't work for both without a mod, but it's kind of manufactured with the same chip for both, so the required mod is small. I guess I will need a soldering iron for the job.
But this looks promising anyway, if I don't manage to get a PS3 MPS for a reasonable price there is another option at least.