How to charge A/C system??

[frankbenz]

Greetings,

I'm new to this forum as I just got my first Benz. It is a 1997 E420.
When I turn the aircondition on, there is a loud hissing sound as if it needs to the recharged. the problem is I was not able to find the low pressure valve of the A/C system. Can anyone point me the location of that valve??? Also the driver's side is always wamer than the passanger side regardless the temperature settings, even at the lowest settings.
Appreciate inputs and advices. Thanks

Frank