Thank you for this question.
Please note that a RadContextMenu property will appear on all controls, but not at the elements accommodated by these controls. To make thing more clear, in this particular scenario RadRibbonBar is a control, while RadButtonElement is just an element inside it.
In order to be able to show a RadContextMenu at a RadButtonElement, handle the MouseDown event of this element, check of the mouse button if the right one and call the Show method of the RadContextMenu component:
sender, MouseEventArgs e)
(e.Button == System.Windows.Forms.MouseButtons.Right)
I hope this helps.
the Telerik team