Sure you're not thinking of HBOGO? The one that's free with a cable subscription? HBONow is a standalone service you pay for monthly, like Netflix. Try hbogo.com/activate.

HBOGo doesn't even offer comcast for Android TV. However, you could use your phone and then cast it via the HBO GO android phone app. That one supports comcast. It's weird, it's supported by virtually all other platforms, but not Android TV with Comcast.