We plan to implement a dialog spellchecker in RadEditor for RadEditor for ASP.NET AJAX, but we are still not ready. Currently, you can use the standalone RadSpell for ASP.NET AJAX control to spellcheck the editor's content as it is shown in the following KB article: Using RadSpell "Prometheus" dialog with RadEditor "Prometheus" as a custom tool.
If you still experience any problems: please, provide sample HTML content and steps to reproduce the problem. Once we are able to reproduce the problem, we will do our best to help.
the Telerik team