Please make sure that your project is targeting ".NET 4.0" profile and not the ".NET 4.0 Client Profile" as RadChart for WinForms does not support the latter.
Hope this helps.
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.