If an order or invoice will be paid with a credit card, you should have an account at Authorize.net to complete the process on SelBuk. Please to activate it on SelBuk do the following
Main Menu > Database
Touch Sales - Invoices
Touch Unpaid Invoices-Orders
Select an invoice
On Payment Method touch Cash > Select Credit Card Now > Done
Then on Account type > touch Authorize.net label to type in the Login and Tran_key given by them > Done