The mega-menu for mobiles was intentionally hidden. Although when viewing the mega-menu on a desktop everything that was added with the page-builder will work, on mobiles it’s not possible to load the resources needed for elements from the page-builder  ( for example sliders/ JavaScript based elements).

A workaround to this would be to add the following css in Theme Options > Advanced Options > Custom css field, but at your own risk.

@media (max-width: 800px) { 
.zn-megaMenuSmartArea-content {display:block !important;} 
.zn-megaMenuSmartArea-content .zn_section > 
.container {width:100% !important} 
.zn-megaMenuSmartArea-content .znListItems {display:block;} 
}