The low pressure side, as nobleman said, is on the right hand side of the vehicle. (from the drivers perspective) There should be two different sizes, so you really cant get it wrong.
On a lighter note, on my girlfriends Grand Am, the high and low pressure ports are under the vehicle on the AC compressor. You actually have to put the vehicle on either jack-stands or ramps to charge the system. Talk about a pain in the butt.
Tom