that trick does not work in the new V3 API that is used by Design Extender.
in the google_javascript folder, open the map JS file.
look for the following code:
// open info window for default address
if (isDefault) {
infowindow.open(map1,marker);
map1.setCenter(point);
}
the reference to map1 may be different in your file, and comment the infowindow.open(map1,marker); line:
// open info window for default address
if (isDefault) {
//infowindow.open(map1,marker);
map1.setCenter(point);
}