Assuming that you are logged in your account, click on "My Account" by hovering over the display pic or your name displayed on the top right corner of the control panel.
From the choice of tabs, pick "API & Integration" by clicking on it. You should now see your API key displayed. If you would like to reset this API key, you can learn how to do that over in this article.
You can start using this API key to build powerful email verification apps or integrate into your existing platform or application. See our Email Verification API calls documentation for more information.