Something

function doAddNew()
{

  /*Open popup window to add new zone, showModalDialog*/
  var vWidth=400;
  var vHeight=300;
  new_window= window.open ("tariffPrefixGroupAddEdit.jsp?p1=0&MO_ID=503&wizID=2",
  "mywindow1","status=1,width="+vWidth+",height="+vHeight);
  //new_window.document.write('<H1>Popup Test!</H1>');
  new_window.moveTo((screen.width-vWidth)/2,(screen.height-vHeight)/2);

  new_window.focus();
}