The fill opening is about midway up on the drivers side of the differential body and requires the same allen tool as does the drain (which is located at the bottom). You can't easily overfill it if the car is relative level because the oil runs out the large fill hole as fast as you could pump it in.

You should purchase a small hand pump, available at most generic auto parts stores for around $5.00 US. It will make the job cleaner and easier.

DanielW