When you purchase the payment plugin, you will receive a zip file called os_eb_stripe.zip. Go to Events Booking -> Payment Plugins to install it as any other payment plugin. See https://eventbookingdoc.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. You can follow instructions at https://stripe.com/docs/keys#api-keys to get API Keys from your Stripe account to enter into payment plugin parameters.