Hello!
Not sure if you've figured this out yet. I had the same problem and this is how I solved it (only moments ago)
I was already signed into PhotoStream but couldn't get my photos to appear.
Under the Internet channel, there is a PhotoStream App. Open that app and sign in if you aren't already. Then give it a few seconds/minutes to load into AppleTV all your photos from PhotoStream. You'll see them all load in automatically.
And that's it. When my AppleTV next went to screen saver my photos appeared. (Of course, I had already selected PhotoStream as my screensaver folder choice.) So you'll need to make that selection under ScreenSaver, obviously.
I'm guessing that each time you have new images in PhotoStream you'd like to add to AppleTV, you'll need to open that App again so that AppleTV brings them in.
But this new feature is fantastic. I've been trying to find an easy way to make my personal photos be my AppleTV screensaver since i got the device, and this addresses that. Tho to WC's comment, I'll be curious to see what happens when there is a photo in there i don't want included in my AppleTV screensaver feed.
I've also not tried removing images from my PhotoStream yet. Not sure how easy/difficult that might be.