According to our Single-User License agreement, we do allow users to install our products on multiple machines, using the same Serial Number to authorize each machine. In order to do this, simply follow the same installation and authorization steps that you used for your very first install.
This is all under the condition however, that each machine belongs to you and is used only by you. As such, only one copy of the software may be running on one machine at a time. Multiple licenses can be purchased if you would like to run a plug-in on more than one system at a time. This is often a good solution for studios and educational institutions with multiple engineers and users.
To offer this convenience, we trust our users to use the service for legitimate reasons, and to respect the terms of our license agreement. For a brief overview of how multiple installations are covered by our single-user EULA, go here. If these conditions are not observed however, we reserve the right to contact our users in order to investigate excessive re-authorizations.
If you are having issues authorizing with your serial number, please contact iZotope Customer Support for assistance.