The Paypal setup page can be accessed by going to the "Options" button from your main dashboard page. Click on "Integrations":

And then click on "Paypal Setup":

Once on the page you will see the following:

To find your Paypal Merchant ID please check out this Paypal article:

https://www.paypal.com/us/smarthelp/article/how-do-i-find-my-secure-merchant-id-on-my-paypal-account-faq3850

Once you have entered your ID and enabled Paypal you will see Paypal as an option when making payments.