You should just need to put it down:
If you haven't actually found the monsters look in the characters/ subsection of entities, you can find the two puppets in the "mindcoral" section...
If you already have found the models, look for the following:
Is the monster slightly stuck in the ground? Move him up a little by turning off the grid while editing (the magnet button on the bottom left).
Did you put any properties on the monster? Set them all to default and then try testing the level with one at a time.
As this is a very new engine to work with we can only go with the things that would of worked in HPL2, but this should work. Hope this helps!
EDIT: When you go into your level's files does it have a map chace? Delete it.
Also have you tried editing the main SOMA folder's resorse.cfg file and include where your mod directory is?