I was under the impression (from PM) that Bill already owns a Kawai CA-63. That he already has its connected to his PC via USB and that he already has Pianoteq standalone (demo) working with the Kawai inputting MIDI and the UR-12 delivering audio back. So clearly the driver for the UR-12 and the UR-12 itself are working fine.
I just can't see any reason why he has been unsuccessful getting audio out of Cubase... unless, does the application require activation
Here is the most recent version of Cubase AI 6 as well as an update for Halion Sonic SE 1.6
Here is the most recent driver for the UR-12 as well as getting started documentation.
There is plenty of information in this thread to have this up and running. At this point, I would suggest (as has been mentioned before) you should start the conversation with the largest body of Cubase users who may have stumbled across your conundrum before. Forums @ Steinberg.net
Cubase AI is no harder or easier to wrap ones head around from the rest of the DAWs. The reason you're toying with Cubase AI is because you have access to it, it's free, and you are already sort of getting to know your way around it, even if you have yet to be successful. If this is something that you want to do, again mentioned before, there is going to be a learning curve up front.