public void AddMarkerGraphics()
{
      ESRI.ArcGIS.Client.Projection.WebMercator mercator = new ESRI.ArcGIS.Client.Projection.WebMercator();
      GraphicsLayer graphicsLayer = myMap.Layers["MyGraphicsLayer"] as GraphicsLayer;
      Graphic graphic = new Graphic()
      {
           Geometry = mercator.FromGeographic(new MapPoint(115.257113, 33.0696150000001)),
           Symbol = LayoutRoot.Resources["DefaultMarkerSymbol"] as Symbol
      };
     graphicsLayer.Graphics.Add(graphic);
 }

这个主要是FromGeographic()方法，传入的是一个4326的坐标系。