next click the menu on the right of your layers palate (in CS it looks like a an arrow with lines next to it.)
OR you can click on the layer menu and select "create clipping mask" on a mac CS 4 the the shortcut is the "option, apple and G keys"
On PSE I believe it is "apple and G" for mac and I am assuming on a PC it would be "Cntrl and G". (if anyone know for sure please feel free to comment!)
That's it. Pretty cool! I love clipping masks. I can do sooooo much with them.
I used this frame in my freebie Quick Page. The kit is our Chillaxin' Collab kit!!
click on pic to download