You have to drop them, then Z them to the circles (remember using the bricks to plug the rat holes near the beginning of the game?). Sometimes it takes a few tries to get them in just the right spot. If you look at the control table, a light will come on for each when it is in place. You can also expect that you might accidentally drop or kick some into the water, so you will have to go fetch them again. Also, they are not locked into place, so don't be surprised if you accidentally move one you have already placed correctly, and you have to place it again.