The Silverlight Contrib License is very clear that all you need to do is include this in a text file in the source code version and you are good to go: http://silverlightcontrib.codeplex.com/license
If you have any issues with the sample and the code, please visit http://kostaschristodoulou.blogspot.com/2011/10/class-extensions.html and dowload again. Michael helped my with some issues and I also included the MSPL file as he also suggested.
Thanks and sorry for any inconvenience.