The CSS selection dropdown is not available in the default rich text editor for Wikis. That is why we have not included it in the RadEditor version. You can add it to the editor toolbar easily by modifying the editor's tools file. The lists tools file is located at
\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\18.104.22.168__1f131a624888eeed\RadControls\Editor\ListToolsFile.xml
Add the ApplyClass tool (<tool name="ApplyClass" />) and it should appear in the editor the next time you load it.
the Telerik team