First go to: https://squareup.com/developers
Set up a developer account
Create New Application
Copy Application ID and Personal Access Token
go to your 3dpc plugin tab on wordpress admin, settings, Checkout, Square and paste Application ID in APP ID and Personal Access Token into Access Token.
Under Location ID, click on the pin, you will see three drop down menus. Select your store location. Save Settings, now you can process credit cards under Square Payments.
Make sure you set up your location in your Square Dashboard.