I came across the same problem and everything kept directing me to sign up for slingtv which includes all the channels not listed on those sites. For an extra 20 a month. Laughing because that is ridiculous after already paying a cable/internet company so much. Use the roku that has the TV without the router.
Anyways the work-around is here http://verysimple.com/content/xfinity/
All you have to do is sign in to your provider, pick your channel in the list and put in the code that roku gives you. Worked like a charm once I opened up the activation window.
I now have the channels my providers provides on my roku.
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