Example: http://www.telerik.com/help/wpf/radrichtextbox-features-selection-mini-toolbar.html

Resolution: 
The Selection Mini Toolbar feature is added in the new version of the control. Please use the RadRichTextEditor instead the RadRichTextBox. 
Please refer to our documentation for more information on the following link: http://www.telerik.com/help/winforms/richtexteditor-ui-for-applying-rich-text-formatting-ribbon-ui-selection-mini-toolbar.html