Sitefinity has upgraded to RadControls for AJAX (previously known as RadControls "Prometheus"). You can replace the reference for RadComboBox.Net2 with Telerik.Web.UI.dll (in case that the version of RadControl Prometheus you are using does not have RadComboBox, because it was introduced with some delay, you can simply replace the control with standard drop-down-box).
I am sorry that we have not updated our sample and thank you for pointing this out. We'll try to get it updated ASAP.
the Telerik team