you should choose image/adjustments/levels (or even better make a new levels adjustmentlayer). In the levels window you see three little eyedropper-buttons (under the ok, cancel, load, ... buttons) click the middle one and double click on the graycard in your image. this sets your graypoint. If you double click on the middle eyedropper you can even set your own target graypoint by the rgb numbers. same goes for the highlight and shadow eyedroppers...