If you have one of the automatic climate control systems, then A/C *should* come on automatically, as long as you're in AUTO mode. A/C won't come on if you're in ECON mode (or OFF, obviously).
If it doesn't come on, then check off other possible causes. Is the A/C fuse good? Can you hear the relay click when you lower the temperature? Worst case, it could just be that your A/C needs recharging.