In order to download the latest version of RadEditor for MOSS you should go to the Downloads section under your account, click on the Visual Studio Extensions, DLLs, Scripts, Source Code, DNN and MOSS wrappers ... link and download the RadEditor for MOSS: (zip, 13 MB) installation. You can see the attached screenshot for more information.
At this point RadEditor for MOSS_5_6_2 arcive contains the old 2009.3.1208.20 version of Telerik.Web.UI.dll. We will change the archive later today so the wsp solution will contain the correct dll.
You can find the upgrade instructions in the following online help articles:
Upgrade Instructions from version 4.x to version 5.x
Upgrading to a newer version of RadEditor for MOSS
the Telerik team