In CS5, the Adjustments Panel was overloaded and clunky. The Properties panel has been introduced as a control center for edits and masking. When any layer has an Adjustment layer or a Mask, it shows up in the Properties panel. For example: If you have an image with an Adjustment layer, the Adjustment layer icon shows up on the left, and the Mask icon is on the right–even if there is no mask.
To add a mask, simply click on the Mask icon on the Properties panel or at the bottom of the Layers panel. If you have a certain area to mask, make a selection first. You can also create a mask from the Refine Edge button. The Properties panel works essentially the same as the old Masks panel. In addition, if you double-click on any mask, the Properties panel appears. Or you can choose Window > Properties.