I assume you are trying to activate/register a product which you have previously registered. Is that correct?
Your Magix Account and all details of your registered products are linked to your email address, apparently, in your case, an old one which you have forgotten. You really do need to try very hard to remember that address, because I suspect that not even Magix staff can help you unless you do remember!
In this situation I think your only option is to email infoservice@magix.net and ask for their help.
OTOH, if this is a new product you have bought which you are attempting to register why not simply create a new Magix Account, using your current email address?