Is your profile signature current? "Aperture 3, Mac OS X (10.7.4)"
A Shared Photostream requires a Mac using OS X Mountain Lion v10.8.2 with Photo 9.4 or later or Aperture 3.4 or later
If you are having Mt. Lion 10.8.2 or later, you need to enable the Shared Streams in the System Preferences > iCloud > Photo Stream > Options first, and then they should be visible in Aperture.