Matrix support confirmed in an email that you must register the game on the 'My Page' area of your Slitherine/Matrix account after the game is released. I am assuming under 'REGISTER SERIAL' option that CMBN will appear in the dropdown.
https://www.slitherine.com/forum/viewtopic.php?f=58&t=93585
My Matrix and Steam accounts are linked, so I see all my Steam purchases on the Matrix 'My Page'.
Seems in these cases the actual mechanics never play out the way the publishers describe, but I'm hoping once the 'REGISTER...' process is complete, CMBN will appear in the Steam library automatically. Fingers crossed.
GeoffP got a reaction from PeterBezemer in CMBN STEAM key