Where is the A/C low pressure port on a 1999 Mercedes E430?

Our Mercedes A/C system stopped blowing cold air recently, so I purchased an "STP A/C Pro Ultra Synthetic A/C Recharge" with a pressure gauge on the dispenser recommended by a local NAPA auto parts store.  The instructions say that the dispenser needs to be connected to the low pressure port, but I am not sure where the low pressure port is on our 1999 E430.

Is the nipple on the silver pipe in the center of the attached picture of the driver's side of the E430 engine compartment the A/C low pressure port?

Possible A/C low pressure port in driver's side of 1999 Mercedes E430 engine compartment