As Jakob Nielsen (the "king of usability") says in his article on mega drop-down menus usability, mega menus have the following characteristics:

• Big, two-dimensional panels divided into groups of navigation options
• Navigation choices structured through layout, typography, and (sometimes) icons
• Everything visible at once — no scrolling
• Vertical or horizontal form factors when activated from top navigation bars; when activated from left-hand navigation, they might appear as mega fly-outs.

How is this menu created?

• Each submenu is a single item
• This item uses the "HTML - Full Mode" option at its "Text" property
• Using full HTML you enter any content you like at the submenu item