Why can't I log in with Comcast XFINITY in Pac-12 Now or Pac-12.com? Pac-12 Support September 07, 2013 • Help / Pac-12 Now /TV Everywhere Comcast / Xfinity subscribers are not yet allowed to watch Pac-12 Networks subscription content via pac-12.com or the Pac-12 Now apps.
http://support.pac-12.com/home
You have to use Xfinity tv to go app from comcast.