1998 Cadillac Deville A/C will not turn on

I stored my '98 Deville for about 6 months and put it on a trickle charger. When I returned six months later the battery was dead so I charged it overnight and had it replaced at Adv Auto Parts and it runs ok. The message center said refrigerant low/ac off. I put some in (about 40 pounds pressure} but the ac still doersn't come on, the message center still says low refridgerant/ac off. When I press the econ/auto button it switches to auto but goes back to econ as soon as I release the button. What do I do to get the ac working? Thanks in advance.