You should see the usage meter at https://customer.xfinity.com/MyServices/Internet/ when you are logged in with your Primary user ID (check with the Lookup tool at https://login.comcast.net/myaccount/lookup).
Click "View data usage details" along the right side of the Services & Equipment page to view the previous 3 months usage.
If you can't find the meter, see http://customer.xfinity.com/help-and-support/internet/do-not-see-usage-meter/.