Telerik Web Parts for SharePoint 2010 last build was R1 2023 SP2 and is discontinued since then.
The CSS class dropdown of RadEditor displays all classes defined in the page or from external CSS files. The dropdown is populated from the CssClassesCollection. By default RadEditor for SP2010 uses the same CSS files as the default rich-text editor. Those files are HtmlEditorTableFormats.css and HtmlEditorCustomStyles.css, which are located in the following folder:
/Program Files/Common Files/Microsoft Shared/web server extensions/14/Template/Layouts/1033/Styles
The folder location can be different if you are not using US English version of SharePoint 2010. The 1033 number signifies the current culture and might be different for different language versions of SP2010.
The files which properties need to be modified are ToolsFile.xml / ListToolsFile.xml, located in the following folder:
/Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/6.x.x.0__1f131a624888eeed/Resources/
<root> <tools name="MainToolbar" enabled="true"> <tool name="ApplyClass" /> </tools> <classes> <class name="Clear Class" value="" /> <class name="Links Class" value="a.link" /> <class name="Images Class" value=".img" /> <class name="My Red Text" value=".redText" /> </classes> </root>