What's the trigger for like a unit stacking on top of another unit, but the units won't move around when stacked. I need this trigger for use of weapon holding.

For Example:
Unit: Civllian
Weapon: Lurker
Lurker is placed in the civillian. The Civillian will move around and such and the lurker will follow the civilian, but it is IN the civillian.

To get what you want, you must stack both the Lurker and the Civilian, note that the lurker cannot burrow, or the stack will be lost, to do this, place the unit as a disabled sprite, instructions on how to do so can be found here.

Now, have a trigger that Enables the doodad state of the disabled sprite, be sure to do this out of the player's vision or they may crash, and then order the unit, and it should be stacked, and stable, if it crashes, chances are that is just a crashing unit, not all of them work.

And on a side note, this topic was made a few days ago. http://www.staredit.net/topic/6301/ (well, a very similar topic; you could have used deductive reasoning to find your answer.)

Use the search function next time, please; it will save you time, as well as others.

Edit: Great, now I'm sounding like that generic annoying guy who pretends to be a moderator.