To show a popup on button or link click on do that from the Link Picker. You have the option to select Popup and select the popup name that you gave when creating the popup. (https://s3-eu-west-1.amazonaws.com/cdn.supporthero.io/article/2618/2c8eb23e-c1fa-4e5f-9e24-aa9cb17f57c7.png)

You can add a Popup by clicking on the pages button -> Add New... and select Popup You will see all the existing popups in your Pages section.