When the car is cool, take off the upper radiator hose where it goes into the radiator. Hold the hose slightly higher than the top of the engine and pour as much coolant as you can get into the hose. Then reconnect it (be careful, the plastic radiator elbows are brittle and have been known to break easily when clamped).

You might have to do this a few times, but it worked for me. I don't know where the bleed screw is on my car either (if there is one).

I agree with Cliffmac, if the coolant system was recently flushed by someone who didn't know better, it is possible that the heads did not get filled with coolant and there are now air pockets in the system. On American cars, the water pump circulates coolant through the heads as soon as you start the engine. Mercedes is different. Do a search on it. I think I read a post here about "burping" the system after flushing or changing hoses.

__________________
1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles