How do you keep units on void terrain? When I place it in the editor and play the map, it says the units are not placed.

place the unit somewhere else, then moveunit on the void

That doesn't seem to work for me.. Does your method include resources like gas tanks and ore chunks?

IIRC, there's only one or two squares of null terrain that can be walked on. They're to the right of the creep tiles in the Tileset Indexed section of SCMD's terrain window.

If you're on installation, the walkable terrain is much harder to get to (though it exists).

Note that as soon as a unit is on walkable terrain, it can walk over anything until it gets to a normal terrain piece.