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 using the tab at the top.
Conversely, to load from iCloud, access the slots from the Load Game screen by selecting the tab.
Do note that if you are using iCloud Drive, all your other devices also need to be using this in order to be able to share between them.