Stop me if I'm wrong :
You want to hook your Wii, into your Arduino, having your Arduino connected to your computer, then use Processing to display the Wii on your computer?
Or, just hooking up the Wii-mote to the Arduino, and control a Processing based game?
http://todbot.com/blog/2008/02/18/wiichuck-wii-nunchuck-adapter-available/
That's an adapter from Wii-mote to Arduino without having to cut the cords. Can also do a search for Arduino Wiimote in Google, you'll find code and everything.