you can try to get a refund from envato because Propulsion will not support a megamenu. We never promised this feature!!!
This page: http://themeforest.net/item/propulsion-responsive-business-ecommerce/1126092?ref=Kriesi does not contain any reference at all.
You can't access the documentation without buying the theme files so obviously you can't expect that the mega menu is part of the package BEFORE you buy the theme. In addition the mega menu is just mentioned one time with the sentence "Setting up the Menus and Mega Menus on your site" and nothing else indicates that this menu supports Mega Menus (detailed instructions, etc. are missing).
The tutorial page states that this videos were made for ANOTHER THEME: http://www.kriesi.at/themes/propulsion/template-files/video-tutorials/ - just compare sentences like "The tutorials for this theme are coming soon, meanwhile check out the ones for the very similar working theme Abundance:" or "Abundance Theme Demo", etc.
In addition all themes with Mega Menu (Brightbox, Broadscope, Abundance) use it on the demo page, Propulsion does not....
Personally I doubt that this argumentation is enough to get a refund...