From the Main Menu, Click on Settings -> Settings -> System Settings.
From the System Settings Menu, Click on 3rd Party Integration.
Click on PayPal.
Check Enable PayPal Integration.
Username: Your PayPal Username.
Password: Your PayPal Password.
Signature: Your PayPal Signature.
Where do I find my PayPal Username, Password and Signature?
You can find your PayPal Username, Password and Signature by logging in to your PayPal account.
If you are unable to find this information, please contact PayPal.