okay...
I think TouchDS is sorrly in need a FAQ's page for this, cause we've been over this before..
but I'll post a quick little note.
This is a PassME:
They are hard to find but you don't need one if you buy the M3 Movie Player (because it's designed for the DS and comes with a PassKey which is very much like this PassME).
The PassKey may or may not store code for the movie player (my guess is that the M3 player it's self carries two sets of firmware (one GBA and one DS)) and the PassKey does little more then "trick" the DS to running the code on the M3 Player...
If you download the firmware hack for the GBA Movie Advance, you would need a PassME (or something else suitable) and you would be limited to running .NDS files below 4MB and (i believe) .GBA files below 256K (because of the systems ram).
With WifiMe you need only follow the instructions provided in the link above. If you have the right wifi card on your PC your good to go, but your limited to using files 4MB and under...
(same as singel cart multiplayer).
and you can not use commercail roms (or so that's the claim).
Also, my apallogise to skabio. What I orginally ment is that there is no need to buy a 'PassMe' (as it that brand name / or product name) because the M3 is designed to run on the DS and has all parts included.