If your coolant is low, top it off with a mix of about 50/50 distilled water to coolant. Only use a proper coolant for these cars, either the Genuine Mercedes coolant or one that is specified and formulated for the application.
It's also a good idea to check the color of the coolant. Very dark or sooty coolant may indicate engine oil is getting mixed into the coolant due to a blown head gasket, or more seriously, cracks in the engine block.