A Mega Menu is a drop-down menu that expands upon a mouse-over event. The mega menu is populated horizontally from left to right with a maximum of three columns. The mega menu is built using the Secondary Menu (Main) 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. The order of the links in the secondary menu dictates the placement in the mega menu. Adjust the hierarchy to get the links in the desired column. For a live example, visit the Drupal Common Framework website.
...
Go to Structure > Menus > Secondary Menu
Click the Delete link under the Operations column
...