Install darkradiant-plugins-darkmod by entering the following commands in the terminal:
sudo apt update sudo apt install darkradiant-plugins-darkmod
DarkMod-specific plugins for DarkRadiant
DarkRadiant is a 3D map and level editor for games based on the idtech4 engines. The software is based on GtkRadiant and focuses on the games DOOM3 and The Dark Mod. Other games based on the same engine, like Quake 4, might be editable too, but are not explicitly supported. . This package provides the plugins necessary to edit levels for The Dark Mod.