When you purchase the payment plugin, you will receive a zip file called os_dms_paypal_pro.zip. Go to Documents Seller -> Payment Plugins to install it as any other payment plugin. See https://dmsdoc.joomservices.com/basic-setup/payment-plugins#install-new-payment-plugin for installation instructions if needed.
Click on the payment plugin to edit, enter payment plugin parameters, then publish it. Follow instructions at https://developer.paypal.com/docs/archive/nvp-soap-api/apiCredentials/#api-signatures to get API Username, API Password, API Signature of your PayPal account to enter into payment plugin parameters.