nation builder isnt saving the api credential information on payment processor page
It sounds like you're seeing the signature and password credential information cleared from their respective fields when you save your payment processor. If that's correct, this is expected behavior.
With valid credentials, you should be able to successfully process a payment from your website's donation page though your PayPal account - even though these fields will appear blank in the payment processor settings of you nation's control panel.
Complete and current information on how to use NationBuilder is available in the documentation section.