An official map editor for Dawn of War: Soulstorm was never released. This package provides all the required attribute and art files necessary to create maps in the Dark Crusade Mission Editor using all the new objects and textures introduced in Soulstorm.
You will first need to have the Dark Crusade Mission Editor installed and set up correctly in order to make use of these assets.
To install this pack, extract the contents of the soulstorm_mapping.zip file to your main Dark Crusade installation folder. Next time you run the Dark Crusade Mission Editor, select File>New and then choose the "Soulstorm Mapping Assets" option from the drop-down menu. If you are then faced with a load of greenish-brown sludge rather than the usual grey tiles, you'll know the pack was installed correctly.
Note that this package only includes the assets for purely environmental objects - any buildings and squads that were introduced in Soulstorm (such as Dark Eldar, Sisters of Battle, flying units, the Deceiver, and some mission-specific buildings like the Imperial Guard's Basilisk Magnus) are not included. If you want to have a map with these kinds of objects, you should use SCaR scripting to spawn them on the map instead.