First basic plugin: methods not triggered, why?

I'm new to Joomla/Hikashop plugin,
I followed the guide at www.hikashop.com/support/62-hikashop-dev...r-documentation.html and i made a plain basic plugin that it doesn't seem to work, the same code put into the history plugin does work instead, probably i made some mistake during the install process:

1. I made a clone folder of /plugins/hikashop/history
2. I renamed this clone folder in /plugins/hikashop/credits
3. I renamed the xml file in /plugins/hikashop/credits/credits.xml
4. I renamed the php file in /plugins/hikashop/credits/credits.php
5. I opened credits.xml changing name tag, description and author tag, and the following tag:

<filename plugin="credits">credits.php</filename>

<?php
defined('_JEXEC') or die('Restricted access');
?><?php
class plgHikashopCredits extends JPlugin
{
	function plgHikashopCredits(&$subject, $config){
		parent::__construct($subject, $config);
	}

	function onAfterOrderUpdate(&$order,&$send_email){
		if(!empty($order->order_id)){
			$db = JFactory::getDbo();
			$query = $db->getQuery(true);
			$columns = array('value');
			$values = array($db->quote('ugo3333'));
			$query
			    ->insert($db->quoteName('les_test'))
			    ->columns($db->quoteName($columns))
			    ->values(implode(',', $values));
			$db->setQuery($query);
			$db->query();
			}			
		return true;
	}	
}

sorry,
in my previous post I wrote that I edited 'credits.php' while instead I als edited 'credits.xml' changing plugin name and reference to php file:
<filename plugin="credits">credits.php</filename>
Plugin folder, XML file and php file have same name 'credits' and XML refers to credits.php file as shown above. Do I have to name the class inside the php file 'credits' too?
Thank you very much for the support!

I'll double check everything when I'll be back to work, hope to find the mistake soon

Hi again nicolas,
I found the problem, it was a stupid syntax php mistake inside my plugin.
Now the plugin works, but I'm not able to catch a particoular event, I need my custom code to be executed when the order has been payed using paypal. My paypal mayment method is set to change the status to 'confirmed' when the order is verified and to 'created' when the status of the order is pending.
I tried both:

function onAfterOrderCreate(&$order,&$send_email) { 
if(!empty($order->order_id) && ($order->order_status == "confirmed")) {
--custom code--- }}

function onAfterOrderUpdate(&$order,&$send_email) { 
if(!empty($order->order_id) && ($order->order_status == "confirmed")) {
--custom code--- }}

Thank you so much,
the event is actually fired, i tested it writing an 'hello world' inside the db.
what doesn't work is the following:
The full $order object seems to be null when the event is fired from paypal, instead it is defined when manually updating the order status.

if(!@include_once(rtrim(JPATH_ADMINISTRATOR,DS).DS.'components'.DS.'com_hikashop'.DS.'helpers'.DS.'helper.php')){ return false; }
$orderClass = hikashop_get('class.order');
$order = $orderClass->loadFullOrder($order->order_id);

Thanks it works perfectly now,
I don't see these information mentioned anywhere in the developer documentation page, i think you should add it as it's very useful.
Thank you again, you have a very good support!