If you would like to use a mega menu, please head to the Appearance > Menus tab in your admin section and make sure that “CSS Classes” are turned on in your screen options tab in the upper right hand corner of your screen. Once the box for them is checked, you’ll see an input field for “CSS Classes (optional)” in each of your menu items.
All you have to enter in that box to make an item the mega menu holder, is “megamenu”. By default it will be a three column mega menu until you add the second class which can be any of the following:
So for example, if you wanted a 4 column mega menu, all you would need to type into your css class box would be “megamenu cols-4” without the quotes of course.