Alright, so the basic issue with this code is that the dummy missile starts off not spawning at the caster unit. There are other obvious flaws(the missile fails to home in, but first the issue with its creation needs to be addressed).
Did you try replacing dat.caster with GetTriggerUnit()? If that works, maybe there is a problem with the preprocessing (check it after it is compiled?).
Preprocessing all seems to be fine. It even bugs out if I have locals with the x/y of the caster.
Sorry for the bump, but if I don't bump it, it leaves first page, and I never get a response(its happened on nearly every other help thread I have ever posted).
Edit: Error isn't in creation, its in the movement. Goofed some X/Y values, lol @ noub.