Both Rating and IsRated are only get methods. This means you can get the value of the rating but you can't set it.
That can only be done by the default media player
For Rating http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.media.song.rating.aspx
For IsRated http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.media.song.israted.aspx
As you can see both only have get methods