OK I just solved my own problem...
1. Go to adobe.com/downloads --> download the trial installer and run it
2. During install, choose the option to install as trial or subscription (as opposed to serialized)
3. Launch Acrobat XI, logging in with valid subscription info when prompted
4. Quit Acrobat
5. Log out of CC desktop app and back on.
Acrobat XI is no installed and added to the CC desktop app as a valid CC app.