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Your ML's A/C compressor is a clutchless unit and you will see the pulley turning at all times. It varies the pressure by use of a "swash plate" instead of a cycling clutch. The swash plate pitch is control by PWM signal from the A/C control unit. If the center bolt on the compressor is turning with the pulley, then the pump is at least turning as well. On the newer vehicles, attempting to just add a little won't work. There are a lot of redundant checks with pressure and temperature, which requires the proper amount of refrigerant to be charged. The fittings are standard R134a which do not require any special adapters. The biggest problem is if a fault has been set in the AAC control unit, even if the fault has been rectified, it must be cleared and rest before the system will begin to function (not all the time, its dependent of what code was set).
The best thing that you can do it take it to a technician that is familiar with Mercedes-Benz vehicles to have it diagnosed and repaired.
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