The PayPal payment gateways use API keys. This document will show how to get Live API keys. The process for getting Test API keys is documented in our PayPal Sandbox doc.
If you haven't already, you will need to make sure your PayPal account is a Business account and not a Personal account. This is required for accessing the API interface.
Creating API Keys
Go to https://www.paypal.com/us/cgi-bin/webscr?cmd=_profile-api-access. Log in with your normal PayPal account. Once you're logged in, find the box labeled 'NVP/SOAP API Integration'. In that box click 'Request API credentials'.
On the next page look for the box labeled 'Request API signature'. Make sure the radio button is checked, and then click 'Agree and Submit'.
The next page will contain API Username, API Password and Signature. Each has a 'Show' link next to it. Clicking 'Show' will reveal the proper string for each item.
Entering Keys in Restrict Content Pro Settings
Once you have those three items, in the WordPress admin go to Restrict > Settings > Payments and enter the three items in the PayPal Settings section.
Getting Test API Keys
Getting Test API keys is documented in our PayPal Sandbox document. Take the Seller keys created there and enter them into the Test API fields you find under the Live API fields on the settings page mentioned above.