I tried your renderer with the following map and it worked for me.
<esri:Map x:Name="MyMap" WrapAround="True" Background="White">
<esri:ArcGISTiledMapServiceLayer ID="PhysicalTiledLayer"
Url="http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer"/>
<esri:GraphicsLayer ID="MyGraphics" Renderer="{StaticResource SelectRendererPoint}" MouseLeftButtonDown="GraphicsLayer_MouseLeftButtonDown">
<esri:GraphicsLayer.Graphics >
<esri:Graphic>
<esri:MapPoint X="-140.9" Y="63.391">
<esri:Geometry.SpatialReference >
<esri:SpatialReference WKID="4326" />
</esri:Geometry.SpatialReference>
</esri:MapPoint>
</esri:Graphic>
</esri:GraphicsLayer.Graphics>
</esri:GraphicsLayer>
</esri:Map>
private void GraphicsLayer_MouseLeftButtonDown(object sender, GraphicMouseButtonEventArgs e)
{
e.Graphic.Selected = !e.Graphic.Selected;
}
Did you set your Graphic.Geometry.SpatialReference? Is GraphicsLayer.Visible? Double check that you've associated the correct symbol with GraphicsLayer that has Point geometry.