Did you try to add it to TV Everywhere by picking "Xfinity" in the list of popular providers? If so, that's not it. The Xfinity source is for Channels DVR users who are subscribed to a cable TV package with Comcast/Xfinity.
At this time, Channels DVR does not support Xfinity NOW. My guess is that it will be added to the list when/if Xfinity NOW works with TVE. Still to be figured out if that will be the case.
Well, for now, are you able to use your NOW credentials to watch live TV on some networks' websites? That will give you an idea if it works with TVE.