Firstly are when you say level do you mean upper/lower level in the same map (game level) or actually trying to teleport to a new map (game level) ??
If it's between maps (game levels) can't be done without a game mod that supports this.
If it is just between levels (layers) in the same map. Make sure you place the name of the entity you are spawning to in the teleporters "IfUsed" property. Not sure you can just use a trigger zone, you need a start and end entity to and you place the teleportifused script in the start entity then place the name of end entity (you want to teleport to) in the "IfUsed" property.
Then try it.