there is a plug on the cylinder head towards the front (14mm if I remember correctly) and you pour coolant until it flows out of there then you put the plug back in and fill the rest of the way.

start the car with the heater on full.

with a completly empty system(drained crankcase) your car should take 9.5 liters of coolant

more info:

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/20-0100.pdf