To purchase credits you will want to head over to https://radiusplus.com/profile/pricing by logging into your account and selecting "add credits: from the drop down in the top right.
Select the plan that is right for you and go to checkout. If you have credit card details on file it will ask you to confirm your details before checkout. If you don't have a credit card on file it will ask for your address and credit card details. Checkout this article to update your default credit card.