You can find information on how to modify the CSS Styles DropDown in the following help article http://www.telerik.com/help/aspnet-ajax/css_styles.html
What you should know is that when you are using RadEditor for MOSS you need to modify the respective ToolsFile.xml or ListToolsFile.xml, located in the Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\Resources
depends on the version of RadEditor for MOSS).
I hope this helps.
All the best,
the Telerik team
, the state of the art learning tool for Telerik products.