The "DJ-MegaMenu system plugin" has to be enabled in order to display additional parameters in menu item edit form. Please go to the Joomla! Plugin Manager, find "DJ-MegaMenu system plugin" and enable it. This plugin is installed with DJ-MegaMenu package, but has to be enabled manually.
If plugin is enabled, but additional option tab is still not displayed then probably your PHP version is less than 5.3 and DJ-MegaMenu version is 1.0.0. In this case you just need to update the menu.