Enter the PayPal partner (usually PayPal). Use the same credentials you would use to log in to https://manager.paypal.com.
Enter your merchant login
Usually this is the same as the merchant login
Enter the password to your PayPal merchant account
This is a test account (see help)
Check this to use test mode instead of running live transactions. To use test mode, you may need to create a sandbox account at https://sandbox.paypal.com and then enter your sandbox email address at http://manager.paypal.com under Service Settings -> Hosted Checkout Pages -> Setup. (Source: https://www.paypalobjects.com/webstatic/en_US/developer/docs/pdf/payflowgateway_guide.pdf 19 July 2013 p.50)