To set up payment methods (to be able to select them on the drop down list, inside any service configuration) you have to go to:
System -> Economic aux. -> Payment Methods
In the next window, you can create a new payment method by clicking New, edit an existing one using the green button, or delete a payment method by clicking the red icon on the right-hand side of each entry.
If you press on Edit, the following window will open:
Here, you can modify the payment method and its underlying type using a dropdown menu.