Thank you for your interest in RadControls for WinForms.
As noted here, RadRichTextBox and RadSpellChecker require .NET Framework 4.0. On the other hand, Visual Studio 2008 can load only assemblies built for .NET 2.0, .NET 3.0 and .NET 3.5 (and compile applications for the same versions of .NET) and this is why VS2008 is not able to load these two assemblies. You need Visual Studio 2010 or above in order to work with RadRichTextBox and RadSpellChecker.
I hope this clarifies the situation. Let me know if you have additional questions.
All the best,
the Telerik team