1) Start by clicking the Account Settings tab in the drop down list under your name in the top right corner.
2) Next click on the Plan & Billing info tab in the left margin or click the blue box on the right named View Plan & Billing Info
3) To change or add your credit card information click the blue button named Change Payment Info.
4) Now you can enter in your credit card information. When you are finished, click the green Add Card button in the bottom left.