I’m not familiar with CS4 but the alpha file should be the same size as its color counterpart... that is 640 x 480, 1024 x 768 or whatever size. Also, the alpha file is usually a simple black & white image or it can also have varying shades of gray.

This being the case open both files in Photoshop. Press “V” to activate the move tool. Select the alpha file and while holding down the Shift key drag it onto the color image and release. Holding the shift key down will center the incoming image over the other. You will notice that there is now a new layer containing your alpha image over the original color file in the layers palette.

Turn off the visibility of the lower (color) layer by clicking the little eye icon in the left column of the layer. Make sure the alpha layer is visible and is highlighted and click on the channel palette tab.

Assuming that the file is RGB you should see the combined RGB channel on top followed by the individual color channels underneath as R, G, B layers. They should all appear to be similar (your alpha image). You can drag anyone of the channel layers (R, G, or B) to the create new channel icon at the bottom of the palette which looks like a little sheet of paper right next to the little trashcan icon (delete). This will now create a replica of your alpha image on a new channel called alpha 1.

Back in the layers palette you can now drag the alpha layer to the delete icon at the bottom since you already have it as a channel. Or you may decide you would like to keep it there for keep sake but disable its visibility.

You are now ready to use your alpha channel. You will find that clicking on either the alpha layer or the alpha channel a selection will be made in accordance to the white of the image. This allows you to subtract that area from the color layer or you can work within only the selected area and do whatever you need do to the color layer. You can also reverse the selection by holding down Shift + Ctrl + I.

This is a quick and dirty tut. There is much you can do but that would be a whole other tut.