In order to activate this code and fill your balance you need to:

Copy the code that you’ve received.
Visit https://hellcase.com/ and sign in with your Steam account
Press ‘Plus’ button on the top right which stands for filling up the balance, paste the code you've received into the Promo code field.
Your balance will be filled up for the sum equal to the price of the Wallet card you've purchased plus the amount's if you reached any.