I started with a new Joomla installation and installed T3 and Purity III (free). I tried using the Megamenu, but it doesn't work. I only have the options "Submenu/Group/Positions/Extra class/Icon/Item caption", the top configuration bar.
I can't find any "Separator/All modules/Layout", or Submenu/Column configuration.
Later I switch to using a blank T3 template, to check if the problem persisted, and it does. The preview doesn't match the frond end view either. It does the correct positioning though.
I checked the online manuals and I think I did everything correct (enabling the Megamenu and all that).
It seems like I'm forgetting some stupid detail, but I can't find it...