In Appearance -> Menu you have to click on Screen Options and then tick CSS Classes. You will have to add the proper css classes into that boxes:

• For home: “menu-icon home”
• For pages: “menu-icon pages dn-submenu-pages”
• For the diet icon: “menu-icon diet”
• For before/after: “menu-icon before-after”
• For team: “menu-icon our-team”
• For recipes book: “menu-icon recipes-book dn-submenu-recipes-book”
• For contact: “menu-icon contact”