On your editor open the file d2keditor.ini (with wordpad or any text editor); there is a section called "[Paths]".
There will appear some lines, if they are wrong you need to write the path where you have the game.
In my case I have it this way.
GameExecutable=D:\Juegos\Gruntmods Studios\Dune 2000\DUNE2000.EXE
That's the route the editor will try to execute the game; probably you have the game in a folder but the editor has a different path.
Check the other three lines too. I am going to copy-paste what I have on my .ini for reference. GamePath=D:\Juegos\Gruntmods Studios\Dune 2000\
GameExecutable=D:\Juegos\Gruntmods Studios\Dune 2000\DUNE2000.EXE
TextUIBPath=D:\Juegos\Gruntmods Studios\Dune 2000\data\UI_DATA\Text.UIB
MissionsPath=D:\Juegos\Gruntmods Studios\Dune 2000\Missions\
Maybe the last line, "missionspath" don't appear on your .ini; but don't worry about that. I think this line appears to me now that I downloaded the .exe provided in this post and after executing the line it's created.
In any case, write the correct path for the lines that appear in your .ini