In case of download version Magix send confirmation e-mail to the address you used in purchasing. If you did not get it check the Spam folder of your e-mail such as, Windows mail, Outlook, Yahoo etc.
OR
If you bought boxed version in that case serial number is on the CD/DVD sleeve (white folder) usually on the back.
Finally if you did not find or get serial number in that case contact the vendor and explain.