Hi,
The IPN URL is "dynamic". The plugin can send an URL which will override the configured one.
But if no IPN url specified, it will use your default one.
It as to be:
http://YOUR_WEBSITE/index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=paypal&tmpl=component&lang=YOUR_LANGUAGE
You have to replace "YOUR_WEBSITE" and "YOUR_LANGUAGE" with the url of your website and your default language code (HikaShop will load the customer language after loading the order, it's just in case for error messages sent by email).
Regards,