Install this free program (7zip), go to the zip file you wanna unzip and right click on it. Select 7zip > Extract here, et voilà: a regular folder containing the image files should appear! (restart your computer if this option doesn’t appear on your right-click menu.) 

I should really make a tutorial, but I’ve been a bit busy, so I’m gonna try my best to explain it.

The basics are: when placing dark textures above the screencaps/art - set them to either ‘lighten’ or ‘screen’, in the layer’s menu. When you use them as background, set the screencap/art to ‘lighten’/’screen’. When placing lighter textures, the same goes, but instead of setting them to lighten/screen, use ‘multiply’ instead, and then play around with their brightness/contrast.

I’ve made some with certain shapes, like dark/light circles. To use them, you can either cut out a subject and place it inside the circle, or, by using PS’s shapes (in CS5 you should find the icon near the text one). Select the circle option, trace one and resize it to fit in with the texture’s circle. Then, in the layers list, place the layer you want above the circle’s layer and then right-click > create clipping mask. Now you can move the screencap around, and you’ll see that’s limited by the shape you linked it to. For the text ones, follow what I explained in the basics part. As for the stock ones, they’re meant to fill backgrounds or negative space.

But this is all relative. Experiment with them, there’s no right or wrong way to use any kind of textures.