here is an example of vec_for_screen:

Code:

---

function mouse_to_level
{
	var vec_from[3];
	var vec_to[3];

	vec_from.x = mouse_pos.x;//can also be your "2d coordinates"
	vec_from.y = mouse_pos.y;//can also be your "2d coordinates"

	vec_from.z = 10;

	vec_set(vec_to,vec_from);
	vec_for_screen(vec_from,camera);

	vec_to.z = 100;//distance_from_screen
	vec_for_screen(vec_to,camera);

	trace_mode = ignore_me + ignore_sprites + ignore_models + ignore_passable;
	return(trace(vec_from,vec_to));
}

function get_target
{
	mouse_to_level();
	ent_create("xxx.mdl,target.x,function);
}

on_mouse_left = get_target();

---

but perhaps its enougth if you use view-enitys?!