I *know* I must be missing something here ...
I installed KeeperFX and the Official editor, and made a very small change to one level (added a creature), saved and exited. To make sure I did it correctly, I copied the level files to the KeeperFX Levels folder. Then I went into the game, to that level and tried to run it.
The creature isn't there. If I go back into the editor, it shows the creature.
What am I doing wrong?