The optional credit card processing routines were added in version 3.0 of SeedCode Complete.
SeedCode Complete is pre-wired to process credit cards using the Plastic Plug-In by 360Works (available separately).
You'll find a credit card logo on the invoicing screen below the invoice, and on the invoice's payments tab:
Click on this logo from either invoices or payments to initiate a credit card transaction. After a successful transaction, SeedCode Complete will create the payment entry for you. (Credit card numbers are not stored in SeedCode Complete.)
Credit card processing requires the Plastic Plug-In from 360Works and a merchant account from Authorize.net