This is my all time favorite layers mask video, by Keith Davis! It's extremely helpful, moves along rapidly, explains 4 different techniques. It's well worth watching more than once, and I find myself referring back to it from time to time.
A few things to remember in working with masks in PSE5:
FIRST, duplicate your original photo. When I refer to "first" photo, I mean the one you're willing to work on and modify, never, ever, your real, original, do-not-want to mess with original photo.
1. You work in the layers pallette. Click on the layer (or photo) you are going to make a mask for. Create an adjustment layer, like "levels." Don't do anything to the levels, just click "ok."
2.Above this adjustment layer, put the NEW background or thing you want to bring into your photo. Now you have 3 layers visible(beyond your background); the top NEW thing, middle adjustment layer with 2 little squares, the bottom first photo or layer.
3.Click letter "d" to change background colors to Black and White. Make black the foreground.
4. Select the adjustment layer with 2 little boxes. Click ctrl>i to fill your right little box with white.
5. Now select your adjustment layer with 2 boxes, but hover your cursor IN BETWEEN the two layers, above the adjustment layer, under the New (top) layer,WHILE HOLDING DOWN the ALT key, until a little weird ball-type thing appears. When it appears, LEFT Click. Your top layer will step aside to the right, with a little arrow pointing down at your adjustment layer (mask). In fact, visible in your work space should now be your FIRST (bottom) photo, not your NEW (top) layer. Your adjustment layer (Mask) is like a window, blocking out your top layer. It's all filled with black; you can't see through a black window. But you can see through a white window, and let the top layer show through like magic!
6. Change your foreground color to white. Select the brush tool. Select your adjustment layer (MASK), (layer with two boxes), and paint with your brush on your photo. You are now letting the very top layer become visible in just the sections you want.
All this sounds much more complicated than it is! Give it a try! If you have questions, leave them for me in the comments!