The sample is not designed to allow adding kml layer from local hard disk. But I guess, it's possible to modify it in order to get a file by 'OpenFileDialog' and to use this file to populate the kml layer.
This is very easily done. Search through the KML sample code and you will eventually find the code that converts a stream object into an XDocument object. You just have to write code to connect an OpenFileDialog.File.OpenRead() (Filestream) object to that routine.