The full version of RadEditor for MOSS is based on the ASP.NET AJAX version of the Telerik's RadEditor. This is the reason why you need to enable Microsoft ASP.NET AJAX in your MOSS site.
Could you confirm that you correctly followed the respective online help article?
Extending your MOSS site with Microsoft ASP.NET AJAX 1.0
Extending your MOSS site with Microsoft ASP.NET AJAX 3.5
Also make sure that you correctly upgraded your version of RadEdtior for MOSS by following the instructions in the following online help article Upgrade Instructions from version 4.x to version 5.x.
If after reviewing the articles the problem continue to exist, provide me with the web.config file of your MOSS site. This way I will be able to try reproducing the issue on our test server.
the Telerik team