The serial number is normally on the disc case, not the product box.
If you have installed the software on a machine already then you might be able to find your serial number thru your Adobe account online.
I don't know if a return to whomever sold it to you can be processed if the box is not available.
If you have not installed the software and the serial number is lost then there is not likely much that can be done except for you to contact Adobe Support using chat and plead your case.
Serial number and activation chat support (non-CC)
http://helpx.adobe.com/x-productkb/global/service1.html ( http://adobe.ly/1aYjbSC )