Please note that ASP.NET and ASP.NET MVC are two completely different frameworks.
Because of this we have created a whole new control family called Telerik Extensions for ASP.NET MVC
I would suggest you use the Telerik ComboBox for ASP.NET MVC
control instead of RadComboBox for ASP.NET AJAX
You can find the key features of the Telerik ComboBox for ASP.NET MVC demonstrated here.
the Telerik team