Obviously you do not know the product you are supporting:
The System.ComponentModel.Composition.dll of version 2009.7.8 is released with the WPF 3.5 version of the controls, as in .NET framework 3.5 there is no System.ComponentModel.Composition assembly. As RadRichTextBox uses MEF for loading the pop-ups that it uses and format providers, we had to implement an equivalent for WPF 3.5.
the Telerik team
If your project targets .NET framework 4.0, you should be using the dlls that have been built for WPF 4 and adding a reference to this Composition assembly would not be required. There are separate zips with hotfix dlls and installers for WPF 3.5 and WPF 4, which you can download from your account.
Now -- where do I get the dll from so that I can see the controls in design mode?