To mark an image in your full-screen slide show--say, for printing or for inclusion in an album--in real time (without stopping the slide show), hit any of keys 1 thru to assign a star rating on the fly.
When you return to the Organizer, simply ctrl-click each of the images that you starred before creating your new album from them.
P.S. It's taken me 3 years of use of PSE7 to become aware of this capability.