This doc give a general run-down of the terrain editor and its controls

Controls:

w,a,s,d - Moves the camera forward, left, backward, and right.
r,f - Moves the camera up and down.
c - Changes camera modes (1-4).
l - Copies all of the previous focus point settings to the current point.
k - Only copies the previous focus point's color to the current point.
= - Toggles the terrain type (normal or water).
o - Toggles paint mode.
- - Normalizes the current point to height zero.
[ - Lowers the current point height.
] - Raises the current point height.
t - Saves the terrain (tempTerrain.txt in the Krig/ directory, not terrains/).


Cameras

There are 4 camera modes:
1 - The normal 'fixed' camera that runs the level script.
2 - A fixed overhead view useful for level editing.
3 - A roaming view that shares the same coordinates as mode 2. Also useful for editing.
4 - An extra roaming camera. Good for setting up a stationary perspective for when editing.