Thank you for the question.
RadRichTextBox component requires .NET Framework 4.0. This is why you can't see it in the Toolbox of your Visual Studio 2008. In order to use it, you have to develop your project in Visual Studio 2010 setting .NET Framework 4.0 as a target framework of the project. We will include a note about this in our documentation accordingly.
I hope that this answers your question.
All the best,
the Telerik team
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