Once you have created a navigation, you can then create a mega menu from one of the existing containers.
To do so, navigate to: System admin > Global navigation. Then click Add Menu and select a Container:

Click Add menu then select Custom and choose the container that you just created:

Add the title of the section of the megamenu and add a title of the mega menu column. Then create a bullet-pointed list of what you want to add, as well as the link and click Add.
Navigate back to the list of containers and click More actions next to the container you originally selected and enable the Megamenu. Then you can add all the relevant columns into the mega menu that you want.
As of our August release, you can now add snippets to the Mega Menu. This provides different text to organisations / user groups based on the permissions of the snippet.
For more information on snippets, please click here.
To add a snippet to your mega menu, navigate to your profile drop down and click System admin:

The System admin screen will be displayed. Within the System admin screen, navigate to System customisation > Global navigation:

The Global navigation screen will be displayed. Click More actions of the mega menu you want to add the snippet to, and click Edit details:

The Edit custom mega menu screen will be displayed. Within this screen, click Snippet link:

The Select snippet screen will be displayed:

Here, you can select and insert one of your existing snippets, which will then be shown within the Custom mega menu screen:

Click Done, once you are done. Your new snippet will be added to the selected mega menu.