Attached is a working example (tested various things so should always work and should keep working).
I use two triggers, one for detecting if the player is inside the trigger radius, the other for ending the mission. Check it out, mission file in the zip - mission takes place at the small Rahrani map.
If you want to use the script, let me know.
EDIT: did the two trigger thing, because I dislike using scripts for things that can be easily accomplished with triggers.
EDIT1: oops, atttached