Renato,
There's a tutorial on the Arduino site that shows exactly how to "read" characters from a PS/2 keyboard.
Keep in mind though, that not all keys can be pressed at the same time... which may defeat the purpose of
building your controller...?
From the Arduino you could emit MIDI, (there's a tutorial there somewhere in the forums.. dont know exactly where)
but if your leaving it plugged into the computer, a serial connection, and a little SuperCollider (or Python if it was my choice)
magic would sort out the MIDI in software...
Good luck, -Harry