There will only be a serial number if you bought the Kontakt Player version. (We had to pay a fee to Native Instruments for that.) If you got the less expensive version of Realivox Blue (which requires the full version of Kontakt), then there is no serial number.
If you did get the Kontakt Player version, then the serial number is in the zip file along with your Instruments folder and Resource Container files.
Posted in: REALIVOX BLUE QUESTIONS