First make sure the number you are using is the serial number and not some other numeric code:
To locate the serial number:
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http://helpx.adobe.com/x-productkb/global/find-serial-number.html
If the issue persists, try contacting Adobe support thru chat.
Serial number and activation chat support (non-CC)
http://helpx.adobe.com/x-productkb/global/service1.html ( http://adobe.ly/1aYjbSC )