If you do not have an account with us, head to https://signup.toucanbox.com/us where you will see the option to enter your code, as shown below:
If you already have an account with us and you wish to apply the code onto your existing subscription, login to your account and head to Manage Subscriptions. You will see your child's subscription listed, along with an option to "Redeem gift or prepay code for this child's subscription". As displayed here: