I have a tutorial using PS Elements 4, might be useful too.

http://www.thebest3d.com/pdp/tutorials/pse4

There is a single file in this case where the transparency mask (or its counterpart the opacity mask) is located in the alpha channel. But I'd assume you can also find this to work with a separate file in a greyscale image holding the mask. Just open both and either mask should become (hopefully) visible in the list when you try to build the mask from the file(s)