First you have to remove the old oil. The outlet is on the bottom part of the gearbox, which is also the lowest part of the engine. Once the old oil is out, try to check is it volume approximately 4 litres.
Inlet for the gearbox oli is on the top of the gearbox, close to its edge and from the back side of the engine.
Access to this inlet is from the bottom of the car.
In order to access it your hand/arm should be not very big. Eventually, first you may remove huge big plastic pipe which will make you easier access to the inlet.
Once the inlet is open, you can put the new oil from the top of the engine using clean plastic garden hose with funnel. The inlet diameter is quite big.
I used 4 bottles of Castrol ATF TQ DIII (1 litre each) for my C5. You can find it in Halfords.
Do not start engine before closing the inlet to the gearbox first, because it is pressurized when engine works.
Good luck.
Gerasim, November 2008
