See post below for adding fluid to the center differential, here's info on adding fluid to the auto trans:
The fill plug will look a little something like this:
If I remember correctly it is on the front of the pain, towards the bottom (silly spot).
Proper filling procedures are as follows:
Get car elevated and level.
Hook up VAG-COM to measure temp of fluid.
Remove trans fill plug.
Pump in fluid until it runs out.
Allow car to warm to 37 degrees.
Shut off engine.
Here's a great write-up on replacing the seal
. It's for the B6, but you get the idea.