Same here. I don't know how to solve this problem, especially if iCloud and iTunes use two separate Apple IDs or email. I have agreed to the terms of service and I can't see how to get photos from my iPhone (signed in to iCloud with one ID) to my Apple TV (using iTunes with another ID.) Very, very frustrating.
I figured out a way to, make it work - is somewhat of a pain. I already had two iTunes accounts with different email addresses. Reason for this is I'm from the US working in the UK and needed accounts with both addresses. Using my iPhone, I deleted the iCloud account which would not work with AppleTV. Then I created a new iCloud account using the other email address. I could not find any other way to do this without using the iPhone.
On the PC I was then able to sign into the new iCloud account.
In AppleTV I went to Photostream and signed in with the new account. All is working fine now. The existing AppleTV account for downloading movies was not changed and is still there. I other words, AppleTV is now using two different accounts, one for Photostream and the second for all other stuff. Will find out soon if can can still download movies.
I only want to use iCloud to have screensaver pictures in AppleTV, so there were no issues with potentially losing my existing data in the Cloud. I'm not sure if this is a real issue.