Excuse me for bad english (French is my mother tongue).
I solved this problem on my Macbook Pro and on my iMac.
1.I closed iPhoto
2. I deactivated 'Photo Stream' under System-Preferences-iCloud.
3. I Open iPhoto
4. In left part of iPhoto just before 'Albums' is the word 'WEB' then near is the word 'Show' (or maybe 'display' because I use the french version of iPhoto).
5. Clic the word 'Show'.
6. In right part you can see now the button 'Activate Photo Stream'.
7. Clic this button, then in the left part, under the word 'WEB' appears the word 'Photo Stream'.
8. Clic the word 'Photo Stream' and now your photo sream is downloaded.
9 Thats all. Nice week-end everybody.