The refrigerant used in your Mercedes is R134a not Freon(R12). The high pressure port is at the front of the engine compartment driver's side the low pressure port is driver's side rear of the engine compartment.
For proper test procedures and charge you need pressure gauges and proper instructions.
Alldata had the workshop manual, technical service bulletins, repair and test procedures, wiring diagrams and more. You get a one year subscription to model year specific information. Its inexpensive and what most independent repair shops use.