CAn someone describe where the low pressure port is for A/C in a W210 E420?

[tmorales]

I have a slow leak presumably from the evaporator and have had to recharge my A/C every three or four months. I used to do this myself on my cars that ran "Freon". Can someone tell me where to find the port to recharge the refrigerant on my E420, and does anyone have an opinion on adding a can of SuperSeal Leak Stop to the system. Thanks!