If you want to work with telerik you have to install the package you want to use...
If you already purchased the product you got some emails to direct you to the download/installation...
If you want to try the product, than download the trial and install it...
For ASP.NET AJAX the trail is here:
http://www.telerik.com/products/aspnet-ajax/download.aspx[
^]
For MVC download from here:
http://www.telerik.com/download/aspnet-mvc[
^]