After installation it says unable to validate this serial number for Acrobat Pro XI. Why? What do I do?

I received my serial number online and went through the installation process. After it was successfully installed I wend to open Adobe Acrobat Pro XI and followed the prompt to enter my serial number and then received this error message "We are unable to validate this serial number for Adobe Acrobat Pro XI. Please Contact Customer Support."

I've read several FAQ and troubleshooting discussions but none have touched specifically on this.

I would have called customer support but did not see a number to call.

Does anyone know what exactly is wrong or what I can do to resolve the issue?