Yes, you need DC to use the mission editor, since it takes all the stuff that it displays from the DC-gamefiles and is so stubborn that you can only open maps that are somewhere inside the DC-root-directory
DoW - Dark Crusade
Dark Crusade Mod Tools
Corsix' Mod Studio
If it is a map that is included in SS you first got to extract it (brotip: Corsix' Mod Studio)
Start Corsix' Mod studio, select "Load DoW:SS Mod" and select "DXP2.module"
Go to Data/scenarios/mp and select the SGB and TGA file of the map you want to take a look at, right-click 'em and choose "Extract this File"
The extracted files will be in the DXP2/Data/scenarios/mp/ folder inside your SS-folder
put it inside the DC-Folder for maps (for example: Dark Crusade/DXP2/Data/scenarios/mp/)
I think this is clear
and open it with the DC-Mission Editor.
Start the ME, click "File", "open", browse to the DC-folder where you put the map (DXP2/Data.. you remember?) and open the SGA-file
Last step: press CTRL+F and CTRL+K to toggle Fog and Sky
Hope it helps!