3rd Gen iPad photostream
In Settings, login to iCloud (under the iCloud tab) and there you will get the option for Photo stream.

You can also go to the Photos tab in Settings and enable Photostream there. If you haven't logged into your iCloud account yet, the switch either won't be there or will prompt for login information.

If you are talking about sharing Journals in iPhoto through iCloud, start by creating a Journal, then tap the Share icon in the upper right corner, tap iCloud and follow the prompts.