The 'asp:ScriptManager' tag is registered in the demo application's web config file. For some reason, the registration line -
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
is not executed in your environment. You should check if the System.Web.Extensions.dll file is present in the bin folder of the web application. Also, you should check your machine's GAC to see if an older version of the file is not installed there.