Here is how to purchase a Keywords Everywhere subscription
You need to install the extension, set up and API key and then purchase from the extension's menu

2
Setup the API Key
Once the extension has been installed, sign up for an API key and set it up.

3
Click the "K" icon at the top right of your browser
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.