How to Add Icon for Menu Items

1- Go to admin > appearance > menus and add a menu item.

2- At top-right of the page click on screen options and make sure CSS Classes is checked.

3- Toggle Publications item at menu and  notice that there is a CSS Classes (optional)field. and type in fa-edit.

fa-edit is the class name of the icon from FontAwesome icon library. You can find other icons at their website and set desired classname as well. for example fa-busfor http://fontawesome.io/icon/bus/

If you need more assistance please don't hesitate to ask for support.