I'm not sure where the serial number for Ozone 8 Elements will be but you do not need to worry about the reference to an iLok ID. iZotope have two different methods of authorising their products, either by serial number or by iLok. You should have a serial number so the iLok reference is irrelevant in your case. (FYI, an iLok is a separately-purchased plug-in dongle which can hold your licence details for software from a range of manufacturers, so allowing you to run the specific software on a range of machines, without the need to deactivate it on one machine in order to activate it (use it) on a second.)
Jeff already commented on the whole iLok situation. So, that would be completely optional. You'll be able to perform the plug-in activation just via an iZotope serial number. In the context of MAGIX audio products that include iZotope's Ozone Elements, you'll be provided with that serial number in one way or another. For the Steam version of the Music Maker Premium edition, you'll be able to look up the serial number by finding the tile for Ozone Elements under the "More features" category in the Music Maker in-app store. Bring up the details page for that store item, and you will see that it will show the status "acquired", and a serial number (including a "Copy" button) will be displayed as part of the item description. Simply copy this from here and paste it into the Ozone Elements window for the plug-in activation.
Hope this helps!
P.S.: Just in case anyone using the "regular" MAGIX version of the Music Maker Premium Edition and is looking for their Ozone Elements serial number is reading this at a later point in time - Please note that this version currently doesn't show the tile for Ozone Elements in its in-app store (for contractual reasons). However, for this version, the Ozone Elements serial number would have been e-mailed, either when you redeemed the 2018/2019 Premium edition serial number in the program, or when you purchased such a Premium edition via the in-app store. So, please look for that e-mail. MAGIX customer service would also be able to help recover this serial number from your account.