how do I enter a customer pre-payment

[Earl Marwil]

I recently upgraded to gnucash 2.6.3 on Fedora 20. I'm having trouble
figuring out how to process a customer pre-payment.

Under the Business menu item, selecting Customer->Process Payment, I
select the customer. No Documents (invoices) are displayed since the
account is current. I enter a payment amount and select a Transfer Account.

There is a warning icon in the lower left corner stating "You must
select at least one document or pre-payment to process." and the OK
button is disabled. Where do I find "pre-payment" to select?