Как я понял, эта программа устанавливается как мод. В комплект идёт новый класс Terrain.ecl
После того, как вы запустите редактор с этим модом, нажмите клавишу тестирования уровня с размещённым Terrain и попадаете в интерфейс редактирования.

1. Start SEd with the terrain editor mod (put +game TerrainEditor to the shortcut, or use the bat in ModsTerrainEditorBin)

2. Load the map you want to have a terrain in

3. Insert TerrainEditable entity to it (from ModsTerrainEditorClasses)

4. Press T (start test game) to edit the terrain

-Press TAB to switch from value adjusting mode to drawing mode and back

-To change values in a terrain, it has to be selected first. To do this, aim the crosshair to it.

-In drawing mode you either modify alpha masks of layers or height of terrain depending which layer you are on (LMB/RMB <=> white/black <=> rise/lower)

-Texture browser uses the virtual tree, so if you want custom textures, insert them to the virtual tree and save it.

-Stretching with non-power of 2 values had some bad effects I think

-Color does nothing

5. Press ESC to exit from editing

6. Press Refresh on terrain to refresh the changes you just made to it.

7. I recommend you to save the map before the next step.

8. When you are done editing, you need to change TerrainEditable into a normal SS 107 supported terrain. That is done by using the "Convert to SS-Compatible" property. Press it, and it will create a Terrain entity exactly in the same place of the TerrainEditable, so press delete right afterwards and you have a SS compatible map! You probably should save to a new world so you can still edit the terrain if you want.

9. When you want to run your map in SS, you'll have to put Terrain.ecl in the Classes directory. So when distributing your map, just include that file (and make sure it goes in the right directory) and it should work fine.