I have the same EXACT question.
I bought my xfinity Home system after watching the online setup videos, as the system appeared to be very expandable and allowed sensors to be added from the web portal very easily.
After having the system installed, I ordered a bunch of brand new sensors off the internet (same exact model and brand as installed by Comast) and I then realized that there is NO WAY to add them to the system!
Will some PLEASE offer me instruction on how to access some sort of "admin" or "programming" mode on the touch screen so that I can add additonal window sensors and motion sensors? I will be happy to PayPal you for the tip and knowledge. Please help.