Nothing appends with a simple plugin

-- url of the page with the problem -- : localhost
-- HikaShop version -- : 2.3.4
-- Joomla version -- : 3.4
-- PHP version -- : 5.3.13

I don't understand, I try to do something with a very simple plugin

class plgHikashopMembership extends JPlugin
{
function plgHikashopMembership(&$subject, $config){
parent::__construct($subject, $config);
if(!isset($this->params)){
$plugin = JPluginHelper::getPlugin('hikashop', 'membership');
if(version_compare(JVERSION,'2.5','<')){
jimport('joomla.html.parameter');
$this->params = new JParameter($plugin->params);
} else {
$this->params = new JRegistry($plugin->params);
}
}
}

function onAfterOrderCreate(&$order,&$send_email){
return $this->onAfterOrderUpdate($order,$send_email);
}

function onAfterOrderUpdate(&$order,&$send_email){
$mainframe = JFactory::getApplication();
$mainframe->enqueueMessage('The customer ');
return true;
}

function onAfterOrderDelete($elements){
$mainframe->enqueueMessage('Order delete ');
return true;
}

function onProductBlocksDisplay(&$product, &$html){
$mainframe->enqueueMessage('Produit id = '.$product->product_id);
return true;
}
}


and nothing appends

the plugin is activate and when I include an error, I have an error, I'm sure is running but no message appears