Set up Immediate Payment Notification (IPN) configuration 
in your Paydotcom account:

Your PDC account -> My Products -> View All My Products -> 
General Info -> IPN URL : 
{$config.root_url}/plugins/payment/paydotcom/ipn.php

Configure your Product ID at aMember CP -> Manage Products ->
under "Edit" link - product ID must be similar with PDC product id.