Actions Allowed for Shared Photos
Just because you can see shared albums in your Source pane doesn't mean you can do everything with their contents that you can do with your own photos.
You can copy shared photos and albums to your Library or albums, print shared photos, play a basic slideshow with shared photos, send shared photos to others via email, and upload shared photos to .Mac Slides.
You cannot edit shared photos in any way, assign keywords and ratings, get photo info, make a new album on the remote Mac, use a shared album to create a book, put shared photos on your Desktop or use them as your screen saver, make an iDVD slideshow, burn them to disc, send them to iWeb, or export them in any way.
Basically, you can only view shared photos; for any action that requires making changes, you must first copy the photos to your Mac.