How to change a cab?

Im aware that its done through CCP, But here is the cab: Jointed Rail EMD widecab, Heres the loco : Built-in NS Dash 9. Im really confused, It worked with another loco of mine.. But not this one, Why not? Do i need a engine spec thats the same as the cab or what?
Am i a total retard for doing this
HELP!

It's the interior container that needs changing. The engine-spec is only for throttle/brake power and DCC speeds. I would clone the Dash 9 first before applying any changes.

that cab only works with payware jointedrail sd70macs
you cant add it to built in dash 9
and on which loco this cab woks for you?

Oops, I mean the C40-8 cab.. It worked on my amtrak stuff but i changed that..Also, I cloned a test verson.