Did you move any files around recently?
The "Landscapeeditor.ini" file in the same folder as the landscape editor program specifies a path to the base of the Black and White 2 game folder. Make sure this file has the correct path. Note that this is the path to the Game folder, not the actual landscape editor folder.
Right-click on the program and select 'run as administrator'.
Like the other person mentioned, you cannot run this and the game at the same time. Occasionally, the game will crash or exit uncleanly and leave a process running. Try restarting the computer first.
-ego533