Please find attached a simple example which demonstrates how to implement validation in RadComboBox for Silverlight.
Telerik.Web.UI.dll assembly is used with our RadControls for ASP.NET AJAX, so you don't need it when using RadControls for Silverlight.
As to the question about licenses - yes, your license is valid, you can login
in your Client.Net account with your email. CustomerID is needed in case you have some problems with the license or the purchase of our products.
Hope this helps.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!