Account credit can originate from several different sources such as promo codes, referral codes, credit for bulk purchases, etc.
To be able to create machines or other resources, you must have a valid payment method on your account even when using a promo code.
Entering Promo Codes
Promo codes are entered on the Billing tab of the console. Referral codes are automatically applied at signup by clicking a link with an appended referral code or they can be entered on the Billing tab before any payments are made.