Assuming the serial number begins with P2- it is unlikely that the product can be registered.
If you wish to try, the download for your registered product is available in your account at magix.com, login using the Account icon top right of the home page, and the email address and password you used to activate Music Maker MX, and select My Products where you should see the download link and serial number.
@Lance-Troian Do your still have the CD? I don't think the downloader will still work.
Activation might be possible if you select offline activation after installing the program and entering the serial number and computer ID into the activation option in your customer account at Magix.com.