I see that you have downloaded the Silverlight 3 version of our controls and I am afraid that URI namespaces ("http://schemas.telerik.com/2008/xaml/presentation") are not supported in Silverlight 3. You could either upgrade to Silverlight 4, which I recommend, or use normal namespace mapping - for example xmlns:telerikNavigation="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Navigation"
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