Not Sure How to Use the CSS selector in PopUp Pro

I am trying to get a popup to work on this page https://www.mindfulscience.es/mi-cuenta-gratis/ (see screenshot #1)

I am using the Divi Theme and have placed the css .dia1 in the appropriate fields(screenshot #2 .. but when I click on it nothing happens

see the settings in the PopUp pro (screenshot #3 )

Not sure how to get this to work, any ideas?