Here is how to purchase a Keywords Everywhere subscription

Before you can buy credits, you need to have installed the Keywords Everywhere Chrome extension or Firefox add-on

Once the extension/add-on has been installed, you need to sign up for an API key and set it up.

In the popup that opens, at the bottom, you will find the link to purchase a subscription. Clicking this link will take you to the pricing page where you can choose the plan you want to buy.