By default the event for the RadContextMenu is RightMouseClick that can't be handled unless your Silverlight application is set to Windowless.
This can be done from the .aspx page of your web project. Just add this line where the silverlight application is configured.: Windowless="True". We have significantly improved our online documentation so you can check a comprehensive overview of this feature in a few days, once we upload it on the web.
Please let me know if you need more help.
the Telerik team