I’m having the same problem with this hanging up and crashing SW when you try and run the post process. Not that clear to me what the MILL.LIB file is supposed to be.
Having the same issue too, no solution yet, I’ve tried installing UPG2 from camworks universal-post-generator page which gives a MILL.LIB, tried setting paths to it with no luck, still crashes solidworks 2021
Anyone found a solution to this solidworks crashing problem ? I cant figure it out 
I tried changing the library path and tried using UPG but I’m first time user in this type of things and did not know what to do .
Download and install UPG-2 from Universal Post Generator | CAMWorks
Open the source file in EControl syntax editor (part of UPG-2 install) - edit line 118 to point to the MILL.LIB file in the UPG-2 install directory. For standard install, on my system it ended up at C:\CAMWorksData\UPG-2\MasterLibraryFiles\MILL.LIB
Open UPG-2, in the file dropdown menu press Compile Source. Find your sourcefile and press the compile button. The .ctl file you need ends up in C:\CAMWorksData\UPG-2\ctl - copy the files to the solidworks post directory
Bumping an older post. Has any further progress been made on this? I downloaded the post processor in this thread and spent the better part of the day working with UPG-2 and SWCam on a project for work. I edited a bunch of the sections in UPG-2 source file and mucked with some of the setting in each of the tabs (2D mill, etc) and have a more trimmed down, not quite as foreign looking g-code for Marlin. Have not introduced tool changes yet. I am still trying to get SW to do what I want it to do. I would be happy to share the modified source file, but I am just hacking away at it since I know almost close to zero on SWCam.