Hey, I'm a member of the CapFrameX team.
Just to clarify, you don't have to add anything to the ProcessGameNameMatchingList.
You can choose a custom name that is displayed throughout CX, in your example ArmA III instead of the original arma3_x64.exe. But you don't have to go to that directory and edit the file, you just set the name in the "Game" line below the list on the left and click on "Save". The text file is only there to store that information. But the naming is completely optional and not required to capture anything.
And you also don't have to select the process in the list in order to capture it. This is only required if you have more than one process in the "running processes" list to let CX know which one you want to be captured.
If there's only one process it automatically gets captured if you press the hotkey.
I think this is explained quite well in the manual I wrote on github.
https://github.com/DevTechProfile/CapFrameX#how-to-make-a-capture
However looking at your case it seems like it's not always as intuitive as we thought it'd be, so we'll be making some small adjustments with the UI to make some things more clear.