The PayPal Sandbox allows you to set up test accounts that can be used to test the full registration process in Restrict Content Pro with the PayPal gateways.
To create a sandbox account, visit http://developer.paypal.com and click Log into Dashboard in the top right. You can log in with your existing PayPal account.
Once logged in, click on the Accounts link under the Sandbox menu.
That will take you to this page:
Click on the blue Create Account button. You will now be presented with an account creation form:
Choose Business for the account type and select your country/region.
Once the account is created, it will be shown in the Accounts list under the Sandbox menu.
Under "Manage Accounts", hover over the "..." and click "View/Edit Account".
This will open a popup window. Click on the API Credentials tab and copy the NVP/SOAP Sandbox API credentials to Restrict > Settings > General, as shown in the setup instructions for PayPal Pro, PayPal Express, and PayPal Standard.
You can log in with your sandbox credentials at https://www.sandbox.paypal.com just like the live PayPal site. This is where you can view your test transactions, set up your sandbox IPN, etc. You can learn more about sandbox testing here: https://developer.paypal.com/docs/classic/lifecycle/sb_overview/