Read the manual. It's with on_level :

Code:
PANEL* pLoadBar = { layer = 9999; }
function on_level_event(percent)
{
  set(pLoadBar,SHOW | LIGHT);   // switch load bar on
  vec_set(pLoadBar.blue,vector(255,200,200));
  pLoadBar.pos_x = 50;
  pLoadBar.pos_y = 50;
  pLoadBar.size_x = 10 + 3 * percent;   // 10..310 pixels
  pLoadBar.size_y = 10;
  if (percent >= 100) {  // level loading finished    wait(-1.0);          // display the load bar for one further second
    reset(pLoadBar,SHOW);  // switch it off
  }
}


djfeeler