An example CPT with the Ninja Forms Append A Form met box.

CPT_Example.php

class CPT_Example {
    public function __construct() {
        add_action( 'init', array( $this, 'custom_post_type' ), 0 );
        add_action('add_meta_boxes', array( $this, 'ninja_forms_add_custom_box' ) );
    }

    public function custom_post_type() {
        //Source: http://generatewp.com/post-type/
        $labels = array(
            'name'                => _x( 'Examples', 'Post Type General Name', 'text_domain' ),
            'singular_name'       => _x( 'Post Example', 'Post Type Singular Name', 'text_domain' ),
            'menu_name'           => __( 'Example CPT', 'text_domain' ),
            'name_admin_bar'      => __( 'Example CPT', 'text_domain' ),
            'parent_item_colon'   => __( 'Parent Item:', 'text_domain' ),
            'all_items'           => __( 'All Items', 'text_domain' ),
            'add_new_item'        => __( 'Add New Item', 'text_domain' ),
            'add_new'             => __( 'Add New', 'text_domain' ),
            'new_item'            => __( 'New Item', 'text_domain' ),
            'edit_item'           => __( 'Edit Item', 'text_domain' ),
            'update_item'         => __( 'Update Item', 'text_domain' ),
            'view_item'           => __( 'View Item', 'text_domain' ),
            'search_items'        => __( 'Search Item', 'text_domain' ),
            'not_found'           => __( 'Not found', 'text_domain' ),
            'not_found_in_trash'  => __( 'Not found in Trash', 'text_domain' ),
        );
        $args = array(
            'label'               => __( 'example', 'text_domain' ),
            'description'         => __( 'Example Custom Post Type', 'text_domain' ),
            'labels'              => $labels,
            'supports'            => array( 'title', 'editor', ),
            'hierarchical'        => false,
            'public'              => true,
            'show_ui'             => true,
            'show_in_menu'        => true,
            'menu_position'       => 5,
            'show_in_admin_bar'   => true,
            'show_in_nav_menus'   => true,
            'can_export'          => true,
            'has_archive'         => true,
            'exclude_from_search' => false,
            'publicly_queryable'  => true,
            'capability_type'     => 'page',
        );

        register_post_type( 'example', $args );

    }

    function ninja_forms_add_custom_box() {
        add_meta_box(
            'ninja_forms_selector',
            __( 'Append A Ninja Form', 'ninja-forms'),
            'ninja_forms_inner_custom_box',
            'example', //Custom Post Type
            'side',
            'low'
        );
    }

}