A Mega Menu is built using the Secondary Menu feature. Follow the Secondary Menu (Main) Knowledge Base article to manually add the links you would like to see in the Mega Menu on your site.
Creating a Mega Menu
Instructions
Go to Structure > Menus > Secondary Menu
Select Add Link
Provide a Menu Link Title
Provide a Path for the menu item
Enter a Description that displays on mouse over
Check the Show as Expanded checkbox
Highlight important information here. Select the editor to edit this panel.
Click Save
Add Comment