This is the motorcycle forum but as I have a car I might be able to help you. I've never seen a dipstick for a gearbox as it's something you would not be checking for oil once a week.
Make sure the car is parked on a flat surface, next there's a filler level plug about half way up the gearbox, undo this & start topping up your oil through the plug hole. A big gearbox like that will probably take at least 4/5 litres of oil.
You will know if the oil level is correct when the new oil starts to seep out from the plug hole. Tighten up the plug & jobs a good un.
This is how the procedure is done for the majority of cars but the Merc may be different, a 2nd opinion would be no harm.
Steo, November 2009
