Assuming you are getting Error code 14 - then you are entering the serial number into the wrong MAGIX product or version/edition
If you had previously installed the program and have re-installed from a new download, and your serial number is for an older version of Photostory then you cannot use in a newer version.
Check you have installed the correct version, also you should be able to get the correct installer by logging into your account at magix.com and selecting My Products - there you will see your registered products.
If you have purchased the new version check the email you received with the serial number. If there was an additional program included then you may have used the wrong serial number, also copy the whole serial number starting with P3- and including the P3- and paste it into the activation dialog.