I am having the same problem. I contacted a Comcast agent and was advised that this service has not yet been set up by Comcast. If you go to the history.com/activate site, there are numerous providers that have enabled their customers to use this service, but apparenlty NOT Comcast.
There is NO cost to Comcast to make arrangements for its customers, and since Comcast is one of the largerst, and supposedly sophisicated, cable tv providers, it is puzzling why Comcast has not yet made arragements for its customers to take advantage of this service. This also aplies to A&E and the Lifetime channels!
I have been a long time Comcast customer, but I have found that its services have been deterioating lately, and this seems to be another good example. The rates continue to go up, while the services decline!