The folder you want should be at:
[path to Documents folder, excluding that folder]\Saved Games\id Software\DOOM 2\WADs\3
So, if your Documents folder is at the default location, then that location would likely be:
C:\Users\[user name]\Saved Games\id Software\DOOM 2\WADs\3
However, if your Documents folders has been moved to, say, "D:\ExtraStuff\Documents", then that location would be:
D:\ExtraStuff\Saved Games\id Software\DOOM 2\WADs\3
Once you find the folder, you should find a file simply named "3"; that's the WAD file you're looking for. Copy that file to your desired location, and then rename the copy.
Hope that helps!
(And in case it's not obvious, you do need to download No Rest for the Living within the Unity version of Doom II first.)