Reading in XML/KML files using C#

You didn't include any code, but I would guess that you forgot to include your namespace when referencing things. Here is an example.

Basic access:

var placemarks = xdoc.Element("kml").Element("Document").Elements("Placemark");

Using namespaces:

var ns = XNamespace.Get("http://earth.google.com/kml/2.2");
var placemarks = xdoc.Element(ns + "kml").Element(ns + "Document").Elements(ns + "Placemark");

My guess is that you've forgotten to use the namespace in your LINQ to XML queries. It's easy enough to extract the data from this:

XNamespace ns = "http://earth.google.com/kml/2.2";
var doc = XDocument.Load("file.xml");
var query = doc.Root
               .Element(ns + "Document")
               .Elements(ns + "Placemark")
               .Select(x => new PlaceMark // I assume you've already got this
                       {
                           Name = x.Element(ns + "name").Value,
                           Description = x.Element(ns + "description").Value,
                           // etc
                       });

If that doesn't help, please post a complete example of what you've tried, and what went wrong.

I used SharmpKml and its documentation to extract information from KML file.

using SharpKml.Dom;
using SharpKml.Engine;
using SharpKml.Dom.GX;

TextReader reader = File.OpenText(filePath);
KmlFile file = KmlFile.Load(reader);
_kml = file.Root as Kml;

sPlaceMarks[] tempPlaceMarks = new sPlaceMarks[1000];
if (_kml != null)
{
  foreach (var placemark in _kml.Flatten().OfType<Placemark>())
  {
  tempPlaceMarks[numOfPlaceMarks].Name = placemark.Name;
  tempPlaceMarks[numOfPlaceMarks].Description = placemark.Description.Text;
  tempPlaceMarks[numOfPlaceMarks].StyleUrl = placemark.StyleUrl;
  tempPlaceMarks[numOfPlaceMarks].point = placemark.Geometry as SharpKml.Dom.Point;
  tempPlaceMarks[numOfPlaceMarks].CoordinateX = tempPlaceMarks[numOfPlaceMarks].point.Coordinate.Longitude;
  tempPlaceMarks[numOfPlaceMarks].CoordinateY = tempPlaceMarks[numOfPlaceMarks].point.Coordinate.Latitude;
  tempPlaceMarks[numOfPlaceMarks].CoordinateZ = tempPlaceMarks[numOfPlaceMarks].point.Coordinate.Altitude;
  numOfPlaceMarks++;
  }

  foreach (var lookAt in _kml.Flatten().OfType<LookAt>())
  {
  Placemark placemark = (Placemark)lookAt.Parent;
  for (int i = 0; i < numOfPlaceMarks; i++)
  {
    if (placemark.Name == tempPlaceMarks[i].Name)
    {
      tempPlaceMarks[i].Name = placemark.Name;
      tempPlaceMarks[i].Description = placemark.Description.Text;
      tempPlaceMarks[i].StyleUrl = placemark.StyleUrl;
      tempPlaceMarks[i].altitude = lookAt.Altitude;
      tempPlaceMarks[i].AltitudeMode =(SharpKml.Dom.GX.AltitudeMode)lookAt.GXAltitudeMode;
      tempPlaceMarks[i].Heading = lookAt.Heading;
      tempPlaceMarks[i].Latitude = lookAt.Latitude;
      tempPlaceMarks[i].Longitude = lookAt.Longitude;
      tempPlaceMarks[i].Range = lookAt.Range;
      tempPlaceMarks[i].Tilt = lookAt.Tilt;
      break;
    }
  }
}