Pop the hood en look down between the fender and the AC radiator. There should be a filling-thingy sticking out.

On my facelift A8 it's located at the hight of the passenger-side headlight, about 5 inches from the headlight towards the middle of the car. The plug stands proud of a piece of tubing in front of the radiator.

At least, that's how it works on the facelift A8 but also on my trusty 1993 Audi 100 V6.

Audi A8 2.5L V6 TDI quattro (2001)