Adds Custom Post Type archives to the WordPress-powered menus. Compatible with WP 3.8

custom-post-type-archive-menu-links.php

<?php
/*
Plugin Name: Custom Post Type Archive Menu Links
Plugin URI: http://codeseekah.com/2012/03/01/custom-post-type-archives-in-wordpress-menus-2/
Description: Easily Add Custom Post Type Archives to the Nav Menus
Version: 1.1
Author: soulseekah
Author URI: http://codeseekah.com
License: GPL2

    Copyright 2012  soulseekah  (twitter: @soulseekah)

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2, as 
    published by the Free Software Foundation.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    This code comes with no guarantees 
*/


// don't load directly
if (!function_exists('is_admin')) {
    header('Status: 403 Forbidden');
    header('HTTP/1.1 403 Forbidden');
    exit();
}


if (!class_exists("Custom_Post_Type_Archive_Menu_Links")) :

class Custom_Post_Type_Archive_Menu_Links {

  /* boot'er up */
  public static function init(){

    // Set-up Action and Filter Hooks
    add_action( 'admin_head-nav-menus.php', array(__CLASS__, 'inject_cpt_archives_menu_meta_box' ));
    add_filter( 'wp_get_nav_menu_items', array(__CLASS__, 'cpt_archive_menu_filter'), 10, 3 );
  }
  
  /* inject cpt archives meta box */
  
  public static function inject_cpt_archives_menu_meta_box() {
    add_meta_box( 'add-cpt', __( 'CPT Archives', 'default' ), array(__CLASS__, 'wp_nav_menu_cpt_archives_meta_box'), 'nav-menus', 'side', 'default' );
  }

  /* render custom post type archives meta box */
  public static function wp_nav_menu_cpt_archives_meta_box() {
    global $nav_menu_selected_id;
    /* get custom post types with archive support */
    $post_types = get_post_types( array( 'show_in_nav_menus' => true, 'has_archive' => true ), 'object' );

    /* hydrate the necessary object properties for the walker */
    foreach ( $post_types as &$post_type ) {
        $post_type->classes = array();
        $post_type->type = $post_type->name;
        $post_type->object_id = $post_type->name;
        $post_type->title = $post_type->labels->name . ' ' . __( 'Archive', 'default' );
        $post_type->object = 'cpt-archive';
        $post_type->menu_item_parent = 0;
        $post_type->url = 0;
        $post_type->target = 0;
        $post_type->attr_title = 0;
        $post_type->xfn = 0;
        $post_type->db_id = 0;
    }


    $walker = new Walker_Nav_Menu_Checklist( array() );

    ?>
    <div id="cpt-archive" class="posttypediv">
      <div id="tabs-panel-cpt-archive" class="tabs-panel tabs-panel-active">
        <ul id="ctp-archive-checklist" class="categorychecklist form-no-clear">
          <?php
            echo walk_nav_menu_tree( array_map('wp_setup_nav_menu_item', $post_types), 0, (object) array( 'walker' => $walker) );
          ?>
        </ul>
      </div><!-- /.tabs-panel -->
    </div>
    <p class="button-controls">
      <span class="add-to-menu">
        <img class="waiting" src="<?php echo esc_url( admin_url( 'images/wpspin_light.gif' ) ); ?>" alt="" />
        <input type="submit"<?php disabled( $nav_menu_selected_id, 0 ); ?> class="button-secondary submit-add-to-menu" value="<?php esc_attr_e('Add to Menu'); ?>" name="add-ctp-archive-menu-item" id="submit-cpt-archive" />
      </span>
    </p>
    <?php
  }


  /* take care of the urls */
  public static function cpt_archive_menu_filter( $items, $menu, $args ) {
    /* alter the URL for cpt-archive objects */
    foreach ( $items as &$item ) {
      if ( $item->object != 'cpt-archive' ) continue;
      $item->url = get_post_type_archive_link( $item->type );
      
      /* set current */
      if ( get_query_var( 'post_type' ) == $item->type ) {
        $item->classes []= 'current-menu-item';
        $item->current = true;
      }
    }

    return $items;
  }


} // end class
endif;

/**
* Launch the whole plugin
*/
if (class_exists("Custom_Post_Type_Archive_Menu_Links")) Custom_Post_Type_Archive_Menu_Links::init(); 

Nicely done. Thanks!

The reason for the error that @julian-weinert and @jeryj (and myself) are experiencing relates to the following: I'm on 4.3 and the related lines are Line 1117 and Line 1121 which reference the $_object variable.

The wp_ajax_add_menu_item() function runs certain checks for the data provided, all of which the plugin passes, but then sets $_object through a switch for the menu-item-type which doesn't account for anything other than "custom", "post_type", or "taxonomy". Meanwhile, this plugin generates this value as the CPT name (see the original under "The AJAX").

Also, as @julian-weinert mentioned, once added to the menu, the link type reads "Custom Link" due to a similar issue in which the off-book usage doesn't trigger anything and falls through to the default labeling.