Shared Photostream and IPad 1 (first generation)
Short Answer: You can't

Shared PhotoStream is a feature of iOS 6 and therefore not available on iOS 5. Some users claim they had gotten the feature working and enabled by removing and re-adding the iCloud account, however I have never seen it work.

From the official description of a shared PhotoStream (Bold by me):


  Select photos directly from the Photos app in iOS 6, iPhoto or Aperture on a Mac