Yep, I'd also have to point you to the external installation. Here's a little more context (and more detailed instructions for you):
The problem you're experiencing is due to a problem with the implementation of the certificate in the Ozone Elements installer MAGIX was provided by iZotope. Until this is addressed via a replacement of the installer on the server, or if you have already lost the ability to trigger another installation through the program due to the previously failed attempt, we would recommend the external installation of the plug-in via the installer hosted by iZotope on their own website.
You can download and install the plug-in via the following link:
(You could already create an iZotope customer account and register your product there at this stage, via the Ozone Elements serial number provided by our system, but you're not obligated to do so in order to be able to download the plug-in installer.)
Once you have installed Ozone Elements on your system this way, please use the VST plug-in path option in the Music Maker program settings ("Folders" tab) to point the program there to pick up the externally installed plug-in. By default, the 32bit version of the plug-in (which is the one you'll need for Music Maker) should be in a location similar to this (but may differ, depending on your installation settings):
C:\Program Files (x86)\Steinberg\Vstplugins\
After this is done, and the plug-in has been scanned and initialised by Music Maker, you'd be able to find it in the program under the effects section, in the "VST FX" sub section.
I was able to install Elements, however I can't register it. I was able to install iZotope Neutrino as well, with their tool and I was able to retrieve the serial key from the store. Elements doesn't appear in the shop since it's part of the premium poroduct.
When you say you weren't able to register it, you mean this is because you simply don't have the serial number, correct?
As a Music Maker Premium 2018/2019 customer, the serial number for Ozone Elements would have been e-mailed to you when you first redeemed your Music Maker Premium serial number (or purchased the Premium edition via the in-app store). So, please look for that e-mail again. Maybe make sure to check your spam filter, too. If you can't get your hands on it, please get in touch with MAGIX customer service, and they'll recover it for you:
(Unlike "Neutrino", Ozone Elements can no longer be listed as an item in the in-app store, for contractual reasons. So, that's why you wouldn't be able to use the same mechanism of bringing up the in-app store info page to look up the respective iZotope serial number at a later point in time. However, the serial is safely store in your account on the MAGIX server, if you ever need to recover it again.)