Hello,
Thank you for the question.
To create “Mega menu” dropdown you need to create a new HTML block in admin panel with Visual Composer (to enable VC for this post type you need to go to Visual Composer -> Role Manager -> Post Types and choose custom then select all post types). You can set it’s layout as you wish. Here is some example from our demo https://docs.google.com/document/d/1n5NuC2NhENQ6FygLzlyAmDz6nTiFH2ryrQ-4ZjfoRYw/edit?usp=sharing
After that you just copy this block’s shortcode (ex.: [html_block id="20396"]), go to Appearance -> Menus and paste it to the desired menu item’s description field. Also change it’s design to “Set sizes” and change dropdown sizes. Check the screenshot
