Year is important. If you have the 722.6xx or 722.9xx you need special MB fluid. 722.6 does not have dipstick, you need special dipstick tool to set level. As stated, car running, in park, on level ground and at operating temp (very important).

5mm hex socket bolt is drain plug.

Without seeing it can't be sure, but I'd vote for a gasket problem. When you get it sorted out make sure someone who knows what they are doing sets the final fluid level.