The most common reason for this is that you have changed your email address since you first installed and registered the program. Could that be the case here?
If so, you need to log in to your accountusing that old address, go to User Data and click on "Change details" (towards the bottom of the page) and then change your registered email address.
Allow at least a few minutes for the change to go through all the Magix systems and then try again to activate the MPEG-s codec.
The only suggestion I can offer in these circumstances is that you email infoservice@magix.net and ask whether there is anything they can do to help you.
Clearly, this is one of those situations where having and using multiple email addresses is NOT a good idea!