After you've opened the Games App and signed into your account, make sure you are on your "My Account" page. You may already be on the account page if you just signed in. If you aren't, then you can tap the menu button on your device and hit the "My Account" button. The location of the Menu button will vary depending on your device, but it is usually at the bottom of the device, next to the HOME button. Consult the user manual that came with your device or contact the manufacturer if you are having trouble locating this button.
Next, scroll down and tap the "Redeem WildCoins" button to be taken to the screen where you will enter your code.
Next, enter your code exactly as it appeared when you received it and hit the green "Redeem" button. Your new WildCoin balance will be shown on your account page.