Q: Wanted to choose Photo Stream in iCloud but it is greyed out and not available - anyone know why?

Wrong version of software?

You really need to give more information  since we only know waht you tell us

see http://www.apple.com/support/icloud/ for more informaiton

LN

I had this problem.  I had to call Applecare.  Finally I spoke to someone high up.  Try this. 

Quit iPhoto.  Go to the user library, then containers, then remove the plists from the com.apple.photostream folders. 

Relaunch iPhoto and go to preferences.  See if you can enable photostream.