You may test Video pro X without a serial number for 7 days, during which time you must extend the trial period to 30 days which I assume is what you are doing.
If so then, the serial number should be in the confirmation email you receive after registering the for the trial extension period with the email address you used to set up your account with Magix.
Check your spam folder for the email - the serial number will begin with TRIALC-