I think Sabreman61 could have been a little more clear with his instructions...
This is the default Kuju ES44AC. The files are now packaged inside this archive:
Assets\Kuju\RailSimulatorUS\RailsimulatorUSAssets.ap
1-
First you have to unpack this archive using RW-Tools or 7zip.
After this is done, you will have this folder in your Railworks installation:
Assets\Kuju\RailSimulatorUS\RailVehicles\Diesel\ES44AC
In this folder you will see other folders including "Black" and "Def2".
Each one is a different livery.
2-
Copy the 4 folders CSX ES40DC, CSX ES40DC (dirty), CSX ES44AH, CSX ES44AH B from the downloaded package into the Assets\Kuju\RailSimulatorUS\RailVehicles\Diesel\ES44AC folder.
Now you have at least 8, maybe more, liveries in the ES44AC folder.
Each one has two folders inside: Engine and Cabview.
3-
Copy es44_Def2.GeoPcDx from Def2\Engine to CSX ES40DC\Engine
Copy es44_Def2.GeoPcDx from Def2\Engine to CSX ES40DC Dirty\Engine
Copy es44ac_cab_Def2.GeoPcDx from Def2\Cabview to CSX ES40DC\Cabview
Copy es44ac_cab_Def2.GeoPcDx from Def2\Cabview to CSX ES40DC Dirty\Cabview
4-
Copy es44ac_black.GeoPcDx from Black\Engine to CSX ES44AH\Engine
Copy es44ac_black.GeoPcDx from Black\Engine to CSX ES44AH B\Engine
Copy es44_cab.GeoPcDx.GeoPcDx from Black\Cabview to CSX ES44AH\Cabview
Copy es44_cab.GeoPcDx.GeoPcDx from Black\Cabview to CSX ES44AH B\Cabview