First, what makes you think it is low on fluid?
In answer to your question, there is a recess in the bottom of your transmission pan with a large allen in the recess.
This is where you fill it by pumping trans.Fluid into the hole.
The car must be level, and up to operating temperature when you fill it.
You fill it until it runs out of the hole with the allen removed.
The proper procedure is to connect a VCDS, and monitor the transmission temperature and fill it at the specified temperature.
I would also strongly suggest you go to your dealer and get factory fluid, as mixing brands would probably not be a good idea.
If your car has close to 100K miles on it, it would be a good idea to change fluid & filter.
2001 A6 4.2 Quattro
1998 A4 2.8 Quattro 5sp.
VAG available-SW Missouri