If you have registered your product with Adobe previously, you will be able to find your serial number after you log into your account. Look for the serial number in the My Accounts section.
If you have not registered your product, get in touch with Adobe Support. You can find more information in this article http://helpx.adobe.com/x-productkb/global/my-serial-number-doesnt-work.html.
This document has a chat link using which you can directly get in touch with support. Do get back in case if you still need more help.