This depends on how you originally bought and paid for the software. If you bought it as a boxed retail version, then it will be on the DVD case; if you bought it as a download then you can retrieve your serial number by logging into the Adobe website using your Adobe ID. If you purchased it through your school or employer then you will need to speak to your network administrator who should have received the relevant licencing information at time of payment.

There is more information on the various methods here:

http://helpx.adobe.com/x-productkb/globa...