This is step 3 from our How to buy Ether tutorial. If you have not read it, do it.
Please note that Coinbase currently only accept Visa, MasterCard, and Maestro cards with 3D Secure authentication support, and charge 3% convenience fees.
When you are logged into your account on Coinbase, go to Settings > Payment Methods page.
Select Credit/Debit Card (3D-Secure)
After you have successfully added your Credit Card (or Debit Card), you can go back to last step, step 4 of the basic tutorial.