In the pop up window that appears, enter the requested billing information - the credit card number, type, expiration date, etc. Then click "+ Save this Card!" to save.
Note that by default the CCV code is not saved, as required by the Payment Card Industry Data Security Standard (PCI DSS). This will not interfere with future charges to the card, as credit card transactions can be processed without a CCV code.
The default billing address is the patient's home address. You can change this by checking the box "Billing address is different from primary home address" in the Billing tab of the Patient Information pop out and adding a billing address.
Once you've saved a credit card to the patient's account, that credit card (identified only by the last four digits of the card number and the expiration date) will appear as the "Current card on file" in the Billing Info tab and can be used to pay charges or add credit to the patient's account.