Did you just download it, or did you actually purchase it first? If you purchased it from the FXhome website, and downloaded it, then the serial number you need will be in one of the emails you received from the company. You might double-check the junk bin of your email account.
The knowledgebase article Where do I find my serial number?
has more specifics if you still can't find it, and if that doesn't help, then you should submit a support ticket so the team can help you track it down.