Most AC systems utilize a low pressure switch that prevents the compressor from engaging if the refrigerant pressure is too low. Given that your mechanic has charged the system already I would suspect a faulty low pressure switch. Your mechanic should be able to jump across the switch - if the compressure engages, then the switch is faulty. THe other possibility is a blown fuse. I am not familiar with your model but many Mercedes after 1984 used a Klima relay that controls the electrical engagement of the compressor. Ask your mechanic if your car has such a relay.