You should find 'redeem code' form under My account in both iOS and Android versions of the app. And it is also available in the desktop launcher too.
If you have redeemed a code that gives you an item, once in-game, check one of the red mailboxes to see if your redeemed item made it to your inbox. Make sure that there is enough space in your inbox because if it is full, you will not receive the item immediately. To do this, please delete a few open letters and see if your item appears.
If you have multiple player accounts, please ensure that you log in with the correct play account where your actual game is in progress. This will prevent you from accidentally redeeming the code with an incorrect player account.