I'm clearly missing something. Just because you have a Cox account doesn't mean all of the channels associated with that account will have a channel on the TV. As for a 'default code' for the TV, there isn't one as far as I know.
There is no code you need... Some AppleTV apps require a cable account to authenticate and that requires simply following the onscreen directions and going to a web site, select your provider and enter requested info on the web site.