In order to pay for a product or service with either credit or debit card you first will have to select the product you want to purchase. Once you have arrived to the payment page it will look like this:
Scroll to the bottom of the page in where you will find what kind of methods you can choose to make payment. Select "Paypal" one, check the box that says "I have read and agree with the terms and services conditions" and click the blue button to process your payment.
This will take you to the Paypal page, in here you will have to either log in to your account or enter your information for your purchase to be processed.
To make the payment click on the "Next" button at the end of the page, and you are done.