You can purchase Credits in two ways:
After creating a new ad campaign, if you don't have enough available credits for your determined budget, you will be asked if you want to purchase more credits.
Credits may also be purchased in the slide-out profile menu by clicking the "Order More" button.
You will be greeted with a window showing you credit purchasing options. You can select either the amount of credits needed to fund the campaign or a bulk credit option below. These bulk options will provide you credits at a discount, so choose whatever option is right for you.
After reviewing your payment method, those new credits will be bought and available for funding your next campaign!