Hi James,
The reason you don't see anything except the text in the T3 Layout Video module is because of the Editor so please turn it off. The following steps show you how to add video for megamenu:
#1: Please turn of the editor: System >> Global configuration, select None for Default Editor.
When done, you can open the T3 Layout Video module, and you will see the youtube video embed code.
#2: Add embed video code to a custom html module (please make sure you publish the module)
<iframe width="560" height="315" src="//www.youtube.com/embed/BhRBVcd-BTQ?list=PL4BjMzuBkvR_N6gu53rkcXEAh7c9iOd77" frameborder="0" allowfullscreen></iframe>
#3: Open megamenu setting panel, enable sub-menu then assign the HTML module for the sub-menu.
Gardner.