PayPal Smart Payment Buttons enabled your site to accept PayPal payment from your customers without leaving your site.
Please follow the video to create a PayPal app and get credentials:
As you have seen in the video, to go the app list:
Get the client ID and Secret:
Now, go to your site Admin Menu >> Downloads >> Settings >> Premium Packages >> Payment tab:
Copy "Client ID" and "Client Secret" from your PayPal app to premium package PayPal config options.
And get your sandbox API credentials from here https://developer.paypal.com/developer/applications/
Finally, click "Save Settings" button and it is ready.
For any technical issue, if you are already using pro version please post in pro forum and free version users please post in free forum. Otherwise, if you have any pre-sale or order related query please contact live chat support team. For technical support.