Diamonds received from the Daily Roll Call are stored in the “Resources” tab of your Inventory. Simply tap on the "Apply" button to add them to your account balance.
If you are playing the game at 00:00 UTC on Day 30, 60, or 90, the Diamonds will be added to your Inventory automatically. However, the notification informing you of your Reward will not appear until the next time you start the game. When you see that message, all Rewards will already have been added.