You don't add paypal buttons to Power Store. Power Store is a shopping cart not a collection of PayPal buttons.
In Power Store, log in to the Power Store Admin section then start adding products to the Power Store catalog.
Also in the admin section, set the payment gateway to use Paypal Payments pro.
When you view thew power store front end pages, you will be able to select the products, and add them to the cart. when you are done shopping, you can select to checkout.
the checkout sequence will connect to paypal to process the transaction.