First, uninstall the Monsters plug-ins from your current machine. During the uninstall, select the option to register that uninstall via the web. As long as the uninstall is successfully registered, you should then be able to install your serial number on the new machine. The license is cross-platform so you can transfer a serial number between Windows and Mac systems in this way. If you own a floating license or if uninstalling is not possible, please contact our support team using the support form to request a license transfer.