How do you put a mission fail trigger and a mission fail message?

Thats a pretty wide question...
Depends how you want to do it

You would probably have a seperate thread waiting until a missionfail message is sent...
The function would finish the mission...

Something like:

mission_fail()
{
     level waittil("missionfailed");  // waiting until notified
     iprintlnbold("Mission Failed."); // message to user
     wait(3);
     missionsuccess("false");
}

This would be controlled by a set of variables like objectives in your map... Doesnt have to be a trigger...

Try looking at some CoD mission files, see how they do it

Ok i will try doing that.