Try their online support: click here. You can send them a message (if you have to) through their system in their link. (For security reasons they may not be prepared to receive messages directly from customers.)
Or, you could try their Subscriptions services troubleshooter - click here.
Are you sure you're keying in the code correctly? Use copy & paste to do this so you can be sure to get it right. BTW, which version of NIS are you using?