What is your MacOS X version? Is your profile current "Mac OS X (10.7.3)"?
The Photo Stream needs
Mac with OS X Mountain Lion v10.8.2 or later and iPhoto 9.4 or Aperture 3.4 or later
or at least
Mac with OS X Lion v10.7.5 or later and iPhoto 9.2.2 or Aperture 3.2.3 or later
if you d not want shared streams.