The above menu has one item: Pop Menu
This has a megamenu grid with 4 image widgets. These link to nothing, but could.
The menu uses a different theme from the top menu, with a white background.
The Shortcode for the above menu is: [maxmegamenu location=max_mega_menu_1]
Here is another example:
There is a recipe on the Mega Menu site that explains how to do this.