ทำได้แล้วนะครับ  ผมเขียนประมาณนี้ครับ

private void searchBttn_Click(object sender, RoutedEventArgs e)
        {
            var input = inVocab.Text;
            XDocument loadData = XDocument.Load("vocab.xml");
            var data = from query in loadData.Descendants("Doc")
                       where query.Element("esearch").Value == input
                       select new Vocabulary
                       {
                           Esearch = (string)query.Element("esearch"),
                           Eentry = (string)query.Element("eentry"),
                           Tentry = (string)query.Element("tentry"),
                           Ecat = (string)query.Element("ecat"),
                           Ethai = (string)query.Element("ethai"),
                           Esyn = (string)query.Element("esyn"),
                           Id = (int)query.Element("id")
                       };
            outputListbox.ItemsSource = data;
        }

ขอบคุณสำหรับความช่วยเหลือครับ