Thank you for the writing.
Indeed, in order to select the last item, you should set the SelectedIndex to the Count minus one. I am glad that you have found the answer to the question.
I would like to let you know that RadListBox control is obsolete since Q2 2010. Once we release Q2 2011, this control will no longer be supported. This is why, it is highly recommendable to upgrade
to its ancestor - RadListControl
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download
; also available is the Q2'11 Roadmap
for Telerik Windows Forms controls.