The Google Play promo code system is a little bit complex to use, it requires you to attempt to purchase the item you want to redeem first. Here are the exact steps to redeem a promo code:
1. Go to the upgrade to pro screen, and click the "Got a Google Play Promo code?" banner at the bottom
2. A dialog will be displayed, please read it carefully.
3. On the next screen, follow the instructions just shown, tap the title, then redeem
4. Verify the google account and enter your promo code