a/c system question

My a/c id not working properly - hot air comes out at times. I looked under hood and see we take the new type of coolant that you can recharge yourself. As I happen to have some with the qage on it, I figured I would check/refill it. I looked under hood and there appears to be two valves: one on the left side (looking at enging from the front) near the firewall and one on right side near the radiator. Both valves seem to look the same and neither is labled. They both are part of cooling system. The one on left side into black box and the one on right goes to a line into compressor. which one to I hook the charging unit up to? Also any ideas what else besides coolant could be the problem? I see no leaks in system and it worked fine last summer. Thanks