To check refrigerant charge level, you need to hook up a/c manifold gauges to read high and low side pressures. If the compressor won't kick in, the best you could get is static pressure readings.
If pressures are low, you need to find the leak. One way to find a leak is to inject a flourescent dye, recharge to proper pressures, then look for the leak with a UV light. Any leak must be fixed first before moving on the refrigerant charging issues. These fixes can get involved....we'll help walk you through these fixes if you feel comfortable enough doing the work.
If pressures look good, then the problem may be electrical.
It is extremely important to charge through the low side port. You could seriously hurt yourself charging through the wrong port.
You also need to know which refrigerant (r12 or HFC 134) is in the system. The port connections should give you a clue.
Do this preliminary work (pressures) and post back.
95 E320 Cabriolet, 131K