Re: I received a voucher
Congratulations on your voucher!
You can redeem the voucher by navigating to your AdWords account's "Billing" tab, selecting the "More Actions" button which is right above the transaction summary page, and hitting the "Apply a Promotional Code". You can add the "code" you received in this section.
If your promotional code is based on a condition - like spend $25, get $100 for example, or any other such criteria, the reward money will be credited to your AdWords account within 5 business days after you satisfy the criteria.
Make sure to apply the code before the expiration date that is mentioned on your voucher.
Do read more about promo codes here: https://support.google.com/adwords/answer/1703648