Make sure the Enable button is green. Click on it to toggle it to either grey or green, but in this case you want it to be green. You can leave the other fields as is, except maybe for the "Available Skin" option. That determines what color the mega menu tab bar will be. The default is Black & White, but here's an example of what the tab bar would look like if you select "Maroon Reddish & Black" from the Available Skin drop down list.

Whatever option you choose, make sure you click on the "Save" button.

On to the next page to work on the menu items.