Get "Key" attribute in XML config file

I have an XML file with a structure and trying to get value from "Key" and "Value":

  <appSettings>
  <add key="Url" value=" http://ift.tt/YK2TsP"/>

  <!--Id's or Xpath-->
  <add key="Origin" value="somevalue"/>
  <add key="Destination" value="somevalue"/>
  <add key="Adult" value ="somevalue" />
  <add key="somevalue"/>

  <!--Controls-->
  <add key="OriginCtrl" value=" Input"/>
  <!--Textbox-->
  <add key="DestinationCtrl" value=" Input"/>
  <add key="AdultCtrl" value=" Select"/>
  <add key="SearchFlightsCtrl " value=" Button"/>

</appSettings> 

I am trying to create a loop that loops through the XML and get value of these "key" and "value". The code I am writing is

            XmlDocument xmlDoc = new XmlDocument();
            XmlNodeList xmlnodelist;        
            string keyname = "";
            string keyvalue = "";
            xmlDoc.Load(filename);
            xmlnodelist = xmlDoc.SelectNodes("appSettings");

            foreach (XmlNode nodes in xmlnodelist)
            {
                keyname = nodes.Attributes.GetNamedItem("key").Value;
                keyvalue = nodes.Attributes.GetNamedItem("value").Value;
            }

The error with "object not set to an instance" rises when i try to run this piece of code. I wonder if there is errors in the code where getting the Value. Any advices would be appreciated.