Recharging the a/c system

I have a 99 leganza, and one day the air went out, So I am trying the recharge the system by putting refrigerant in. I hooked up the can to the hose, and then the hose to the a/c line. and turned the air on full blast. I then turned the screw to puncture the refrigerant can, and nothing happens. The refrigerant is not going into the system. I dont know why. any suggestions or comments?

Also, wich a/c line is the low pressure? the line on top or bottom? thanks.