All event rewards are delivered through Animation Throwdown's mail system.
To access your mail select the blue tab in the bottom left corner and tap the envelope icon.
From your list select your event completion message and tap the yellow claim button at the bottom. A crate will appear and reward you the prize.
If the only message option available is a delete button this means the mail message either did not come with a prize or the prize has already been claimed. Due to how the system works it is not possible to delete a reward message without receiving your reward first.
For more information, see the Mailbox article.