Hello,
First of all, It is necessary to create an HTML block separately for each menu item. Dashboard > HTML blocks > Add new. HTML block is a WordPress custom post type with WP Bakery page builder option allowing create any template.
Megamenu block is created by means of the Extra Menu list element. At the same time, Megamenu block can contain any other static content element such as Product Grid, Images, Texts blocks.
As soon as Megamenu blocks are ready, they may be added to menu items in Appearance > Menu > Select menu > enter menu item > insert HTML Block shortcode into Description in the Custom fields [for theme] which appear as soon as the menu item has been saved.
You need to set “Full-width” or “Set size” in the Design filed in order to get the option of HTML blocks drop-down.
Best Regards