depends on where you bought it but generally the serial number will be in an email that you receive when purchased.
if you bought from a shop sometimes the serial is in your account in the shop like JRRShop.
you need to setup a iZotope account and then register your serial number in that account. Been a while but you might have to activate in that account not sure.
I have an email that detailed all the process to register my iZotope products when I bought them, you should have received something similar.