As TheGrimReefer says, you have to raise the tiles from the floor below.
A little tip is to have 3-4 blocks (The craftable ones, found under stairs), stack them on top of each other, and place a tile above. Lock the tile in place and remove the blocks, saves a lot of clicking to raise the tiles. You dont even have to lock down the blocks when doing this.
If you can catch me here on stratics, or icq, I can also give a hand in-game if you still have problems.