To add font ICONS in menu (top and main menu):
- Go to Plugins -> Add new and install Font Awesome 4 Menus plugin.
- To preview all available Font Awesome icons go here. Click on an icon that you want to use and then copy the code of the icon.
- Go to Appearance -> Menus and select a menu that you want to edit and paste the icon code in a menu item, then save the menu.
- If you want to add only an icon in menu(without text) add this td-only-icon to the icon code for a better alignment.