This archive contains the DNF: Enhanced scripts source code and the level editor, follow the installation instructions on the zip readme file. Also, make a copy of your DNF installation and put it in a directory like "C:\Duke4", the editor doesn't like long paths with spaces.
Update 06/30/2023:
The archive has been updated to match the 1.5.1 release, install version 1.5.1 first in order to use the level editor. The archive also includes the decompiled DNF 2011 UnrealScripts, so you can use them as a reference to make mods. Just to let you know, those scripts can't be re-compiled, they are only here for reference.
Thanks! How did you get this?