Now that you have configured one or more merchant gateways, you need to associate them with credit card vendors. You can have different credit card brands processed with different processing centers. For example, you can configure Parallels H-Sphere to process VISA and MASTERCARD with AuthorizeNet, and AMERICAN EXPRESS - with CyberCash.
To set the associations:
Make sure to create associations for each credit card brand you would like to support. Users won't be able to pay with the credit cards that haven't been associated with any processing center.