I have setup ‘testimonials’ CPT.
Testimonials does show in Screen Options menu, and when selected I can choose to show individual testimonial posts in a menu.
BUT usually there is also a CTP tickbox in the Screen Options menu… that allows you to select the relevant CPT archive page to be added to the menu. This is what I don’t have.
I could always add it as a custom link, but I know it’s supposed to be there! Please help!!! 
CPT was adeed as a mu-plugin using:
<?php
/* ========================================================================================================================
Custom Post Types
http://codex.wordpress.org/Function_Reference/register_post_type
======================================================================================================================== */
add_action( 'init', 'codex_testimonials_init' );
function codex_testimonials_init() {
$labels = array(
'name' => __( 'Testimonials' ),
'singular_name' => __( 'Testimonial' ),
'menu_name' => __( 'Testimonials' ),
'name_admin_bar' => __( 'Testimonials' ),
'add_new' => __( 'Add Testimonial' ),
'add_new_item' => __( 'Add New Testimonial' ),
'new_item' => __( 'New Testimonial' ),
'edit_item' => __( 'Edit Testimonial' ),
'view_item' => __( 'View Testimonial' ),
'all_items' => __( 'All Testimonials' ),
'search_items' => __( 'Search Testimonials' ),
'parent_item_colon' => __( 'Parent Testimonial:' ),
'not_found' => __( 'No Testimonials Found' ),
'not_found_in_trash' => __( 'No Testimonials Found in Trash' ),
);
$args = array(
'labels' => $labels,
'public' => true,
'publicly_queryable' => true,
'show_ui' => true,
'show_in_menu' => true,
'show_in_nav_menus' => true,
'query_var' => true,
'rewrite' => array( 'slug' => 'testimonials' ),
'capability_type' => 'post',
'has_archive' => true,
'hierarchical' => false,
'menu_position' => null,
'supports' => array( 'title', 'editor', 'author', 'thumbnail', 'page-attributes','post-formats' )
);
register_post_type( 'testimonials', $args );
}