You can view the various methods of Adding Funds to your Account from within your Reseller Control Panel. You need to follow the steps mentioned below:
Login to your Control Panel. See details
In the Menu, point to My Billing and click Add Funds.
You would be presented with various methods available to Add Funds to your Account with 1'st Domain Name Service. You can choose one of the options and Add Funds to your Account.
Adding Funds to your account does not automatically settle any Invoice/Debit Note pending against your Account. You will have to manually settle it.