I can not understand how to use migrate_extras and for example how to use geofield migrate extra support.

I have create a Migration class that work. In my "source" I have 2 "fields" called latitudine and longitudine. I have follow the instruction that I have found in geofiled.inc: 

    $geo_arguments = array(
      'lat' => array('source_field' => 'latitudine'),
      'lon' => array('source_field' => 'longitudine'),
    );
    // The geometry type should be passed in as the primary value.
    $this->addFieldMapping('field_coordinate', 'Point')->arguments($geo_arguments);
    // Since the excerpt is mapped via an argument, add a null mapping so it's
    // not flagged as unmapped.
    $this->addFieldMapping(NULL, 'latitudine');
    $this->addFieldMapping(NULL, 'longitudine');

But my "test node" is created (via migrate) without "geofileld" field (field_coordinate). But if I comment the lines above and insert this function: 

    public function prepare($node, stdClass $current_row) {
      $node->field_coordinate = array(LANGUAGE_NONE => array(0 => array(
        'geom' => 'POINT ('.$current_row->longitudine." ".$current_row->latitudine.')',
        'geo_type' => 'point',
        'lat' => $current_row->latitudine,
                 'lon' => $current_row->longitudine,
                 'left' => $current_row->longitudine,
                 'top' => $current_row->latitudine,
                 'right' =>$current_row->longitudine,
                'bottom' => $current_row->latitudine,
       )));
    }

"geofield field" is "created". 


Where is my error?