Promo codes are entered upon checkout, and are currently only available for use on the app version of PangoBooks.
When you are ready to buy a book, proceed to the checkout screen, where you should see a box labeled "Promo Code" where you can enter your code. This will add the value of the promo to your PangoBucks! Make sure to click "PangoBucks" as your payment method at checkout so that the promo value is applied.
Please note, promo codes do not discount the price of your order - they add credit to your account!