The class you've linked to is for PHP 4. If you're using PHP 4, obviously you should upgrade now. PHP 5 users should use the real deal, SimpleXML. Here's an example for PHP's SimpleXML. If you have questions about SimpleXML (PHP's, not CodeIgniter's knockoff) read the manual's examples a dozen times and if it still doesn't come to you, please post a new question.
$Comprobante = simplexml_load_file('yourfile.xml');
echo $Comprobante->Emisor['nombre'];
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