I really suggest contacting the theme developer. This is a very easy fix for him/her.
If you absolutely insist, then you should find the kleo_custom_menu
function that is filtering wp_edit_nav_menu_walker
and figure out the name of the theme’s Custom Walker. Then track down that class and copy the code somewhere for backup.
Then replace the insides of the class with the insides of my class Walker_Nav_Menu_Edit_Roles
which is in nav-menu-roles/inc/class.Walker_Nav_Menu_Edit_Roles
. Or you could copy the insides of the original WP class Walker_Nav_Menu_Edit
which is in /wp-admin/includes/nav-menu.php
and then add the hook from the FAQ in yourself after the description input, but I’ve already done that in my class so you may as well copy me.
Then you need to go back to the stuff you copied and figure out which inputs are being added then. You can add them to the wp_nav_menu_item_custom_fields
action hook. You can see how I am doing exactly that in the nav-menu-roles.php
file.