Up to version 8, PS Elements didn't have Layer Masks of their own. Two ways round this.....
a. a slightly complicated method of making an Adjustment Layer (which has its own Mask) and placing it in the right position to act as a Layer Mask...... e.g. Duplicate your image layer; then go back to the Background 'layer' and make a Levels Adjustment Layer - but make no adjustments. So you have, top to bottom, a duplicate, a Levels Adjustment Layer and the Background Layer. Highlight the Levels layer, hold down the Alt button and hover your mouse between the Levels layer and the duplicate image layer above it. The pointer will change to a chain link. Click your mouse. You have now 'clipped' the Levels Adjustment Layer (and, more importantly, its Layer Mask) to the Duplicate image. Whatever you do in the top layer can now be masked (by painting with black) in the Mask (when the mask box is highlighted and you paint on the main image window).
b. Download a Layer Mask Action -
e.g. from here - follow the installation instructions carefully. You can then activate a Layer Mask on any layer you want, easy peasy.