To add a payment method to your account you should login to your account and then click in the left menu on Billing. As indicated with the red arrow in the screen below.
In the next screen you can fill in your Billing details which that will be included on the Invoice. For non-EU customers you do not need to fill in anything at the VAT field. Below these fields you can click the blue button to add a new payment method. See the red arrow below.