This add-on can be treated as both a WP plugin and a theme include.
- Copy the 'advanced-custom-fields-nav-menu-field' folder into your plugins folder
- Activate the plugin via the Plugins admin page
- Copy the 'advanced-custom-fields-nav-menu-field' folder into your theme folder (can use sub folders). You can place the folder anywhere inside the 'wp-content' directory
- Edit your functions.php file and add the code below (Make sure the path is correct to include the nav-menu-v4.php file)