ah ok thank you , I will try that, I copied a piece of code without really looking at the meta boxes.

I used it as a plugin and added the same folder structure to the plugin. So not in the theme itself.

the code that is in the mainfile of the plugin:

add_filter('piklist_post_types', 'onderwerp_post_type');
function onderwerp_post_type($post_types)
{
    $labels = array(
        'singular_name'     => 'onderwerp',
        'search_items'      => __( 'Zoek Onderwerpen' ),
        'all_items'         => __( 'Alle Onderwerpen' ),
        'add_new'           => __('nieuw onderwerp'),
        'add_new_item'      => __('nieuw onderwerp'),
        'new_item'          => __('nieuw onderwerp'),
        'view_item'         => __('Bekijk onderwerp '),
        'edit_item'         => __( 'Wijzig onderwerp' ),
        'menu_name'         => __( 'Onderwerpen' ),
        'not_found'         => __('Niet gevonden'),
        'not_found_in_trash' => __('Niet gevonden in prullenbak'),
        'parent_item_colon' => __('Hoofd onderwerp'),
        'menu_name'         => __('Onderwerpen')
    );
    $post_types['onderwerp'] = array(
        'labels' => $labels //piklist('post_type_labels', 'Onderwerp')
    ,'title' => __('Voeg een titel toe')
    ,'public' => true
    ,'rewrite' => array(
            'slug' => 'onderwerp'
        )
    ,'supports' => array(
            'author'
        ,'revisions'
        )
    ,'hide_meta_box' => array(
            'slug'
        ,'author'
        ,'revisions'
        ,'comments'
        ,'commentstatus'
        )
    );
    return $post_types;
}