Just in case someone else has the same question....
You go to the payments tab for the reservation, and click on add a credit card:
Then in the pop up window, you can add the credit card info.
Once added, you can process the payment due using that card. You can actually enter multiple credit cards if you want to use different ones for different payments. That's how I handle it when a guest want to share the payment with another guest. I enter multiple payments due on the same date in the quote, and then use different cards to make each payment.
Hope that helps!