To create or edit a Mega Menu choose Structure > TB mega Menu under Administration toolbar. (
To make a specific block visible under the TB Mega Menu "Blocks" dropdown list, you have to place it in one of the theme's regions. You can also disable the block so it will not appear on your page.
Let's say you have a view that shows three most popular products in the Store, and you want to place it in the Mega Menu, under the Shop menu item.
The video below will help you to understand how to use TB Mega Menu.
Note that the video is referring to the Drupal 7 module, which is different than Drupal 8. However, the toolbox area, the TB Mega Menu simulator, and the way of menu creation are almost identical in both modules.