For licensing problems, including Activation and S/N's, I strongly recommend contacting Adobe Customer Support (not Technical Support), and have all info on hand, when you call. For them, it should be an easy fix.
I have CS5 Master Collection and it has TWO serial numbers... one for the main install, and a different number just for Acrobat
Since you did not state your version, I do not know if you should have one or two serial numbers
In the US - Adobe Install Problems 800-642-3623
In the US - Adobe Activation 866-772-3623 Open 24/7