What Graham said. It's a feature exclusive to the "Premium" edition of Music Maker.
Just to make this a little more transparent: For iZotope Ozone Elements, you don't need to redeem any credit in order to obtain the license. This item will already be donated to your account the moment you either redeem a the Music Maker Premium edition (2018, 2019 or 2020) serial number within the program, or when you purchase such a Premium edition via the in-app store system (in which case no serial number will be involved or needed). This will automatically make the plug-in installer available for download/installation, and will result in your iZotope serial number to be generated on our server and sent to you via e-mail, so you could then later use it in the Ozone Elements plug-in interface.