Note: the sub-title feature is not included in all themes! To add the sub-title to a menu entry, please go to WordPress admin -> Appearance -> Menus. Then click the “Screen Options” tab on the top right to open the settings panel for the menu page, please check the checkbox for “Description”. Now you can add/edit the menu sub-title, see also the screenshot below. Please note: mostly the sub-title is working only for the top menu entries.