Since you added the location to appSettings using the key "pushPin" you want to use the same key to retrieve that app setting. You also need to cast it to GeoCoordinate, not 'var'.
private GeoCoordinate MyGeoPosition;
private void PhoneApplicationPage_Loaded_1(object sender, RoutedEventArgs e)
{
MyGeoPosition = (GeoCoordinate)appSettings["pushPin"];
}