To accept credit card payment online via WHMCS, a payment processing company will need to be selected. A merchant account through a payment processing company will allow you to accept credit cards such as MasterCard, Visa, Discover Network, American Express, Diners Club International and JCB.
For a great tutorial on how to setup Payment Gateways within WHMCS, please click here. This will walk you through the method of adding the PayPal payment gateway.
Once you have selected a payment gateway, you will need to set a merchant account with them. Please refer to the following article for more information regarding merchant accounts:
After you have a merchant account, you may add it to your WHMCS as a payment gateway using the same method while selecting the account you have signed up for.