When standing behind the tractor looking forward, it is just in front of the left axle housing near the bottom. The plug is over 2 inches in diameter and you will need a 7/8" socket to remove it. The plug is made from a type of plastic, so I recommend a 6 point socket.
Actually from the picture I just found, maybe I'm wrong. It looks like it is on the left side in the same area I mentioned. The screen is #7 in picture and plug is #11. On a 6400, the plug to remove is #5.