Creating an xml parsing function in C#

I have an XML that's obtained from a web service, i'm using an HttpClient for it. This is what the XML looks like:

<respuesta>
  <entrada>
    <rut>7059099</rut>
    <dv>9</dv>
</entrada>
<status>
  <code>OK</code>
  <descrip>Persona tiene ficha, ok</descrip>
</status>
<ficha>
  <folio>3204525</folio>
  <ptje>7714</ptje>
  <fec_aplic>20080714</fec_aplic>
  <num_integ>2</num_integ>
  <comuna>08205</comuna>
  <parentesco>1</parentesco>
  <fec_puntaje>20070101</fec_puntaje>
  <personas>
    <persona>
      <run>7059099</run>
      <dv>9</dv>
      <nombres>JOSE SANTOS</nombres>
      <ape1>ONATE</ape1>
      <ape2>FERNANDEZ</ape2>
      <fec_nac>19521101</fec_nac>
      <sexo>M</sexo>
      <parentesco>1</parentesco>
    </persona>
    <persona>
      <run>8353907</run>
      <dv>0</dv>
      <nombres>JUANA DEL TRANSITO</nombres>
      <ape1>MEDINA</ape1>
      <ape2>ROA</ape2>
      <fec_nac>19560815</fec_nac>
      <sexo>F</sexo>
      <parentesco>2</parentesco>
    </persona>
  </personas>
</ficha>

I'm trying to make a function that can parse this and, right now (just for the purpose of testing my understanding of the language since i'm new to it) i just need it to find the VALUE inside an "rut" tag, the first one, or something like that. More precisely I need to find a value inside the XML and return it, so i can show it on a label that's on my .aspx page. The code of my parsing function looks like this:

public static String parseXml(String xmlStr, String tag)
    {
        String valor; 
        using (XmlReader r = XmlReader.Create(new StringReader(xmlStr)))
        {
            try
            {
                r.ReadToFollowing(tag);
                r.MoveToContent();
                valor = r.Value;
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message, ex.InnerException); 
            }
        }
        return valor;
    }

This code is based on an example I found on youtube made by the guys from microsoft where they "explain" how to use the parser.

Also, this function is being called from inside one of the tasks of the HttpClient, this is it:

protected void rutBTN_Click(object sender, EventArgs e) 
    {                                                       
        if (rutTB.Text != "")                               
        {
            HttpClient client = new HttpClient();
            String xmlString = "";
            String text = "";
            var byteArray = Encoding.ASCII.GetBytes("*******:*******"); //WebService's server authentication
            client.BaseAddress = new Uri("http://ift.tt/11PZiu7");
            var par = "mod_perl/xml/fps-by-rut?rut=" + rutTB.Text;
            client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Convert.ToBase64String(byteArray));
            client.GetAsync(par).ContinueWith(
                (requestTask) =>
                    {
                        HttpResponseMessage resp = requestTask.Result;  
                        try
                        {
                            resp.EnsureSuccessStatusCode();
                            XmlDocument xmlResp = new XmlDocument();    
                            requestTask.Result.Content.ReadAsStreamAsync().ContinueWith(
                            (streamTask) =>
                            {
                                xmlResp.Load(streamTask.Result);             
                                text = xmlResp.InnerXml.ToString();
                                xmlString = parseXml(text, "rut");  //HERE I'm calling the parsing function, and i'm passing the whole innerXml to it, and the string "rut", so it searches for this tag.     
                                Console.WriteLine("BP");
                            }).Wait();
                        }
                        catch (Exception ex)
                        {
                            throw new Exception(ex.Message, ex.InnerException);
                        } 
                    }).Wait();
            testLBL.Text = xmlString;  //Finally THIS is the label i want to show the "rut" tag's value to be shown.
            testLBL.Visible = true;
        }
        else
        {
            testLBL.Text = "You must enter an RUT number";
            testLBL.Visible = true;
        }
    }

The problem is that when i put some breakpoints into the parsing function i can see that it's receiving correctly the innerxml string (as a string) but it's not finding the tag called "rut", or rather not finding anything at all, since it's returning an empty string ("").

I know that maybe this is not the correct way to parse an xmlDocument, so if someone can help me out with a tutorial or some XmlParsing 101 course (for C# on .NET 4.0) i'd be really really thankful. I have searched a lot through the internet and can only find "questions" on "working stuff"; my stuff is not working yet and I'm not totally sure where to start.