You can view your earned Product Credit on the Profile page in your Virtual Office. If you're on a computer, click on your name at the top right and click the View Profile button. If you're on a mobile device, tap on the Menu button at the top right, then go to Account > View Profile. Once on the Profile page, click the Product Credits tab to view your Product Credit balance and history.
If you would like to use your Product Credit, check the "I would like to use my Product Credits for this order" checkbox on the payment page during the VO checkout process.