To use the iCloud functionality, please make sure you have iCloud set up on your device and that you are logged in.
From the Save Game page, you can access the iCloud save slots either by using the Bookmarks Menu button in the title bar or by swiping. The subtitle of the page indicates whether you are currently viewing local or iCloud save slots
Conversely, to load from iCloud, access the slots from the Load Game screen either by using the Bookmarks Menu button in the title bar or by swiping.
Do note that if you are running iOS 8 and have upgraded one device to iCloud Drive, all your other devices must also be updated to be able to share between them.