I click "Mist" in the placeable selector bar. I then click where i want to place it on the area. It just doesn't do anything. No error message.
EDIT: It isn't just the mist. It is other random things like... most of the secret objects but not all of them, a cheese vat, field crops 1, a music sheet...
I have a feeling it is something really stupid that i'll feel like an idiot for not knowing.