Hi Andrew Goodwin,
By default RadEditor for MOSS comes with the default MOSS link manager enabled. You can find the declaration of the tool in the respective ToolsFile.xml or ListToolsFile.xml, which are located in the /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/4.x.x.x__1f131a624888eeed/RadControls/Editor folder.
You should check the moss documentation for information about this dialog.
If you want to enable the dialog provided by Telerik, add the following code to the ToolsFile.xml and ListToolsFile.xml.
I attached the user manuals for RadEditor for ASP.NET AJAX and for the classic ASP.NET editor. You can find information about hyperlinks there.
All the best,
the Telerik team