The first thing to do is to access the global JApplication object, then get a reference to the JPathway object.
$breadcrumbs = &$mainframe->getPathWay();
?>Adding a breadcrumb is now as simple as calling the JPathway::addItem() method. The addItem method takes a name as the first parameter and a link as the second parameter.
$breadcrumbs->addItem( 'My Crumb!', JRoute::_('index.php?option=com_mycrumb') );
If your Item is the last item on the breadcrumb the link will be ignored.