With an existing order open, click on "Add" in the "Payments" section. A dialog will appear for you to enter the payment information. The first option is a drop-down menu of payment types. The types of payment that appear here depend on the Payment Methods you have configured.
If you would like to add a Credit Card payment through your credit card payment provider, select "Credit Card". See "Processing a Credit Card" for more information.
If you would like to simply log a credit card payment to the order WITHOUT using your credit card payment provider, select "Credit Card (Bypass Payment Provider)". This will completely ignore your credit card payment provider and will not actually charge a credit card regardless whether credit card payments already exist on the order or not. The "Credit Card (Bypass Payment Provider)" option functions the same way as "Cheque" or any other payment method that does not use an external payment provider.
Adding a payment in the past
If you add a payment with a date in the past, it means that you've already received the money and so the payment will always be added as "Successful". You cannot add a "Pending" payment in the past.
You cannot add payments of type "Credit Card" with a date in the past. Your credit card payment provider does not accept payments with a date in the past. However, you can add a payment of type "Credit Card (Bypass Payment Provider)" in the past since it does not use your payment provider and simply logs it to the order.
Adding a payment in the future
You can add payments with a date in the future. They will appear as "Pending". For payments of type "Credit Card", this will schedule a payment with your credit card payment provider. For more information on credit card payments in the future, see "Scheduling Automatic Credit Card Payments".
For more information about post-dated payments, see "Post-Dated Cheques (and other pending payment types)".