elements only visible on mobile…

I am trying to have elements only show up on desktop and mobile and I am having trouble. I did research and followed the instructions.

First I checked my functions.php. for;

add_theme_support(‘avia_template_builder_custom_css’);

It was already there.

So I went to add Custom CSS and added;

/*Show only in mobile or desktop*/

@media only screen and (min-width: 769px) {
.only_mobile { display: none !important; }}

@media only screen and (max-width: 768px) {
.only_desktop { display: none !important; }}

I still cant figure out who to add the custom class to the element.. The screenshot that you have on this URL is different for me.

I am not seeing the custom CSS box in the elements.