Be aware that you will have to pay a monthly fee to use that app.
CBS All Access is not offered on the X1 version of the Prime app. It is only offered on the stand alone version, such as the version available on any smart TV or an Amazon device.
"However, HBO, Cinemax and CBS All Access have restricted their channels through Prime Video on Comcast X1 Xfinity devices. That’s because Amazon’s agreements with HBO and CBS disallow access to their programming through third-party providers."
This is a quote from an article about when Prime was first offered on the X1 back a couple months ago.....one would think that an official Comcast employee would know this.....