I'm not sure exactly how you mean that. There is an xfinty app in the roku channel store that you can add in order to stream Comcast's TV. Or if you mean a Comcast cable box, just plug it into an available port, and select that port. (It's essentially "just a TV" in that respect.) Perhaps you should do both!
Or maybe you mean Comcast internet? In that case you need to tell the TV the SSID and passphrase for your router.