If you have the "Pay as you go" payment plan and you have ran out of credit points or you need more, you can order them like this:
Click in the upper right corner, next to your name, on the "Credits" button.
A pop-up window with different credit amounts opens.
Every email costs one credit point. So for example, if you want to send an email to 5000 subscribers, you need 5000 credit points. The minimum amount you can order is 2000 credit points.
Click on "Order now!" next to the amount of credit points you would like to order.
After a little while, the credits will be added to your account. You will receive an invoice for the order to your email (the email that is under "Billing preferences" in your account settings).