Secondary menu can’t be imported because WP does not support custom menu options. To create secondary menu go to Appearance > Menus > create menu and choose as secondary in Theme Locations. Mega menu works almost the same way for the secondary menu as for the main menu but in vertical design http://xstore.helpscoutdocs.com/article/27-mega-menu
To import Electronics demo go to Theme Options > Import/Export > Dummy Content > and press import. Use Additional Pages Import below to import all the additional pages of the electronics demo. If you have already trashed electronics home page then you need to use manual import and go to Tools > Import > WP > upload electronics version .xml file and import. Then import Slider (Revolution Slider > Import).
