It appears that PayPal Express is available in numerous countries not listed under Settings > Payment Processor. Is it that the NationBuilder PayPal integration only works with those countries?
Nation owners can set up PayPal accounts in these countries to be used as your nation's payment processor.
Donors and customers on your website can use major credit cards issued in any country when you're using Paypal as your payment processor.
Complete and current information on how to use NationBuilder is available in the documentation section.