What do you mean by not being able to pick them up? If you see them in game, just press "G". What is happening when you're trying to pick one up? If they are there on your map they should work without any problems.
and how did you place them?
Well, I know that, please explain how did you place them exactly. Step by step.
You can't place stuff like that. It needs to be placed like an object spawner.