menu link to custom post_type?

ok – got this working with a walker in the end;
(added a custom directors link to the menu)

class Director_nav_links extends Walker_Nav_Menu {
    function start_el(&$output, $item, $depth, $args) {
        global $wp_query;
        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value="";

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
        $class_names=" class="" . esc_attr( $class_names ) . '"';

        $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'> ';

        $attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target ) ? ' target="' . esc_attr( $item->target ) .'"' : '';
        $attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) .'"' : '';

        //  automatically add wp folder to url
        if(strtolower($item->post_title) == 'directors' ) $item->url = get_post_type_archive_link('directors');

        $attributes .= ! empty( $item->url ) ? ' href="' . esc_attr( $item->url ) .'"' : '';
        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '<br /><span class="sub">' . $item->description . '</span>';
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }

}