This diagramme is blurry enough to be almost useless, but it'll point you in the general direction - the pulley and its adjuster. Take a look at that mechanism and you'll see which screw to adjust. Tightening increases tension from what I recall and can see. You'll be able to tell. Careful with the belts.
"You just remove the belt cover (two bolts) and then loosen the locknut and adjust the slider bolt to adjust the tension. Proper tension is achieved when the belt deflects 9-10mm with a pressure of 10kg in the middle of the long leg of the belt (the main pulley to A/C compressor stretch). It's pretty obvious when you remove the belt cover and see the setup."
"10kg is about 22 lbs. So push your thumb against the belt, hard (about enough to lift 3 gallons of milk), and the belt should move 9-10mm (three-eights of an inch).
IMHO, better too loose than too tight. If it's too loose, it'll slip and squeal and you'll have to tighten it, no big deal. If it's too tight, you can ruin the bearings in the accessory drive or the A/C compressor. "
Do you guys normally hear a high pitched air sound sometimes when you turn on the A/C or the compressor kicks in? It's not loud enough to be a "squeal" and you probably wouldn't hear it if the radio is on - kind of like the air being squeezed out of a bag for a second, but high pitch - I haven't heard of this sound before on any other car A/C.