Annotum's design only supports two menu levels in the top navigation menu bar: the visible to-level items and one level below that. Sub-sub menus are not supported; you can add them in the menu editor, but they won't display on the site.
It is possible to enable additional menu levels (to display them) by modifying the "depth" parameter set in the 'functions.php' file (wp-content/themes/annotum-base/functions.php); you can set it to 3 or 4 to show additional menu levels. However the 'deeper' levels won't display any differently than the first submenu level, because the Annotum style sheet doesn't provide a specific format for the additional levels. You can modify the style sheet to support that, if desired.
Please note that any customizations to the theme would need to be re-applied if you ever upgrade to a new version of Annotum.