Caution: Because the AdWord promotions' validity largely depended on time, you want to apply the Promotional code if you are sure you will deposit the stated amount (for manual payments) or enable the automatic payment the same day the code was applied.
1. Login to your AdWord Account (assuming you have already setup a Billing Profile)
2. Click on the gear icon on the top right and select "Billing"
3. Click on "Transaction History"
4. Lookup 'Manage Promotional codes
5. Click on the "+PROMOTIONAL CODE" red button
6. Paste your code in the field supplied and then click on "Save"