SC2Mapster

Use the "point is pathable" function to test if it is pathable to ground units.

point random_pathable_location() {
    point p;

    repeat forever {
        p = random point in entire map;
        if ( p is passable ) {
            return p;
        }
}

This function will return a pathable point on the map.

I'm assuming you're using the GUI. point is passable and repeat forever are both listed in the GUI if you search for them. I'm not sure what the function names would be in Galaxy, but you can do the same.