Please make sure that you did not install the Lite version of RadEditor for MOSS. In this version of the RadEditor for MOSS the Full Screen command is not supported. Please make sure that you install the trial version of RadEditor for MOSS 5.4.1, which is the latest released version based on the RadEditor for ASP.NET AJAX control. For your convenience I attache a file where you can see the differences between different versions of RadEditor for MOSS.
the Telerik team