You can try by turning off the content filters of the RadEditor. In order to do so you need to modify the respective ConfigFile.xml or ListConfigFile.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.
Just add the following code.
More information on Content filters you can find in the following help article http://www.telerik.com/help/aspnet-ajax/contentfilters.html
I hope this helps.
the Telerik team