use vec_for_screen:

Code:
 

function BuildBuilding()
{
   var vecSpawnPos[3];

   if(mouse_left)
   {
       vec_set(vecSpawnPos, vector( mouse_pos.x,mouse_pos.y, camera.z));
       vec_for_screen(vecSpawnPos, camera);
       ent_create( "witch.MDL", vecSpawnPos, NULL );
   }
}


i guess you call this funcion after you clicked on a button, dont you?
so you propably have to you a while loop. :p


nipx