I had a post on this awhile back, here it is:http://larian.com/forums/ubbthreads...s=onuseaction&Search=true#Post615299
Not particularly detailed so let me know if you need some details filled in.
That post was aimed at going between two areas on the same level, not a level transition. But the principles are the same.
The Level you are teleporting to needs a point trigger on the other end that's global. I usually name them based on the level and function.
Generate definitions in the story editor.
The door needs an "OnUseAction" which is set to teleport. When setting it to teleport, pick trigger and select the trigger in the other level/area you placed before. Now you should be done.
Don't forget to do the inverse, door on the new level, trigger on the old.
EDIT: Oh sort of a tip, MAKE SURE that the object you are using for transition (door, sign, whatever) has indestructible stats.