I downloaded a copy of Acrobat Pro XI. During installation it asked for the serial number. I provided the serial number. After installation, at first launch, it comes up asking for the serial number again but this time for a selection from a list of Acrobat apps. Acrobat Pro XI is not in the list. How do I activate my Acrobat Pro XI?
This means your serial number is for an upgrade license, not a full one, so you have to provide an earlier s/n for a full version in order to be able to use it. Are you aware that Acrobat XI is an old version, no longer supported by Adobe?