Paste Into Layer Mask
There are two methods you can use to do this in Elements 3. Hidden Power tools for Elements 4 offer even more options. Either of these can be applied to an image where you have created a layer mask.
1. Choose Highlight Mask from the Hidden Power tools (this will create a selection).
2. Be sure the layer mask thumbnail is active in the layers palette.
3. Fill with Black (Edit>Fill Selection).
1. Select All (with the layer you want to use as a mask active in the layers palette)
3. Create a layer mask (Choose Layer Mask from the Hidden power tools).
4. Option-click / Alt-click [mac/pc] the layer mask.
The second method is more useful if you are copying color layer information to paste into the mask, but the first is quicker and easier for Elements 3.