Follow the pipe work from the condenser back towards the right hand side of the engine bay. If you need to refill the system don't bother with refill kits from parts shops they're a waste of money, you can get it done properly for very little money these days.
the thing is that so many people and garages just top it up and say its done... this just annoys the hell out of me!! They should really ever need refilling - for them to need to be refilled they must've leaked.
What should be done is the entire system emptied with a vacuum pump and then they should leave it for 30min to an hour and see if it holds a vacuum first. If it holds a vacuum for that long then they should refill it and stick some UV dye in and run the system on full for a while and then go inspecting with a UV lamp - there has to be a leak somewhere!!! Just so annoying as I have had so many cars with AC that doesn't work properly - like my bravo now, going through the history its been regassed countless times but cannot find a receipt anywhere for someone actually trying to find out why its been refilled so many times. On top of all that if it has lost gas then its nigh on 100% it will have some air and thus water in the system which even further reduces efficiency - If a car is not relatively new I would be swapping out the drier as well if i was trying to repair and not working or not working well system - yet another reason to never bother with diy kits. And don't even get me started on the environmental issues of people untrained or qualified to mess with refrigerants - you could like leak a bit of refrigerant and got the same kinda effect on the atmosphere as running your house for a year, probably more. I am as it happens at this moment in time writing a air-conditioning inspection report for a leisure centre, i do actually know a bit about this stuff
Anyhow, i'll stop ranting now, with regard to where the low pressure schrader valve is i am not actually sure on but its the one on the fatter of the 2 refrigerant pipes. This is possibly the most common point where they leak as a side point, the schrader valves - they don't change the schraders on tyres when they stick new tyres on for a laugh