Dark basic it's for making your own games & stuffs, if you want to make mod you must use the game engine and change graphics and code variables.
Ex: Medal of Honor's (use Quake 3 TA engine)
to edit I you need to open ".pk3" (it's a zip file renamed) and replace:
- textures ".dds", ".tga"
- models and animation ".skd", ".skc"
- sounds ".mp3"
- maps ".bsp"
by your own stuffs and after that you change the codes from ".tik" adding more bullets, changing weapons name, less gravity, more health, etc.
COD use Q3 engine too, so it's almost the same way to edit MOH...