The RadEditor assemblies are placed in the GAC. If you want to reference them from a user control or a web form (.aspx) file, you need to use the fully qualified assembly names in the Register line. For example:
<%@ Register TagPrefix="rade" Namespace="Telerik.WebControls" Assembly="RadEditor.Net2, Version=188.8.131.52, Culture=neutral, PublicKeyToken=852c9eb6525c1b53" %>
This way you can use the editor control in your pages.
If you need to access RadEditor from the codebehind, then you will need to add a reference to the assembly in your Visual Studio project. Unfortunately, the RadEditor.Net2 assembly is not available by default in the Add Reference... dialog in VS 2005. However, you can manually extract the file from the solution (RadEditorMOSS.wsp). Rename the RadEditorMOSS.wsp to RadEditorMOSS.cab. This will allow you to open the file with windows explorer or an archiving program. Extract the dll files from the cabinet and use them in your Visual Studio 2005 solution. If you want to use the editor in multiple projects, you can copy the assemblies in the VS PublicAssemblies folder (typically c:\Program Files\Microsoft Visual Studio 8\Common7\IDE\PublicAssemblies\). This will make them available in the Add Reference dialog.
All the best,
the Telerik team