wp_list_table bulk action button is not working – no submit

I have a custom wp_list_table to create my own table. Almost everything works perfect, but the bulk action button seems to be not working. It’s proper displayed,but on click nothing happens. I think somehow there is a form-wrapper missing or just some javascript? I added the button described on wp-engineeers: http://wpengineer.com/2426/wp_list_table-a-step-by-step-guide/#bulk

My Bulk-action functions:

function get_bulk_actions() {
    $actions = array(
        'activate'    => __('Show'),
        'deactivate'  => __('Hide')
    );
    //Add delete function for admins
    if(current_user_can( 'delete_published_posts' )){
        $actions['delete'] = __('Delete');
    }
    return $actions;
 }

    function process_bulk_action() {

        $action = $this->current_action();
        if( 'activate'===$action ) {

          foreach($_GET['wp_list_event'] as $event) {
                echo($event['title']);
            }

        }
        if( 'deactivate'===$action ) {
          wp_die('Items deactivated (or they would be if we had items to deactivate)!');
        }
        //Detect when a bulk action is being triggered...
        if( 'delete'===$action ) {
          wp_die('Items deleted (or they would be if we had items to delete)!');
        }
    } 

The rendered HTML (not wrapped by any form tag)

<div class="alignleft actions">
  <select name='action'>
    <option value='-1' selected='selected'>Aktion wählen</option>
    <option value='activate'>Anzeigen</option>
    <option value='deactivate'>Verstecken</option>
  </select>
  <input type="submit" name="" id="doaction" class="button action" value="Übernehmen"  />
</div>