Is your Profile signature correct Mac OS X (10.7.5) ? Then you cannot create Shared Photo Streams. They require OS X Mountain Lion v10.8.2 and Aperture 3.4, see:
You can create Shared Photo Streams with:
- an iPhone, iPad, or iPod touch using iOS 6
- a Mac using OS X Mountain Lion v10.8.2 with Photo 9.4 or later or Aperture 3.4 or later
- a PC using Windows 8, Windows 7, or Vista with the iCloud Control Panel 2.0 or later installed. (iCloud Control Panel 2.1 required with Windows 8.)
If your system is compatible with shared streams, check, if you have enabled shared streams in both the System Preferences and Aperture's Preferences.