Retriving field value
The safest way is to use field_get_items() like this (which will handle any languages):

$xmlPath = '';
if ($items = field_get_items('node', $node, 'field_3d_model_xml_path')) {
  $xmlPath = $items[0]['value'];
}


If you want to access it like you currently are (sometimes it is useful to do so but in your case I don't think so, this is just to be more complete), use the LANGUAGE_NONE constant for the 'und' language instead of hard coding it, like this:

$xmlPath = DRUPAL_ROOT . "/" . $node-&gt;field_3d_model_xml_path[LANGUAGE_NONE][0]['value'];