Try to add Tap
event handler to your map control :
<Controls:Map x:Name="MyMap" Tap="MyMap_Tap"/>
Then you can use ConvertViewportPointToGeoCoordinate
method to convert tapped point to coordinate :
private void MyMap_Tap(object sender, System.Windows.Input.GestureEventArgs e)
{
GeoCoordinate location = MyMap.ConvertViewportPointToGeoCoordinate(e.GetPosition(MyMap));
MessageBox.Show("latitude : {1}, longitude : {0}",
location.Latitude,
location.Longitude);
}