From what I understand, with the 3d ready dlp sets, if you are trying to hook up the xbox to a display that has the 3d sync port on the back you just use the hdmi cable from the xbox to the display. Then the emitter for the compatible glasses plugs in the back of the display (3d port). Your 3d ready game (Avatar) needs to be set to 3d mode and the display switched to 3d mode.
I'm not sure if this applies to your plasma though.