how do I sign into my CBS all access subscription inside the new Apple TV app on my iMac?

CBS All Access is not the same on AppleTV app as it is via browser

or smart tv. It is a totally separate subscription that is purchased via

the iTunes Store. It is the same if you have Amazon Prime as well

and some of the other media provider services.

Sorry, but that is just the way it is (probably has to do with licensing issues)

until CBS releases a CBS All Access app or when macOS begins to support

iDevice apps (CBS does have an app for iDevices that directly accesses your

account).