Make sure you have the Microsoft .Net 3.5 Framework installed on your servers, and place the following in each Web Application’s web.config file under configuration/system.web/pages:
<controls>
<add tagPrefix=”asp” namespace=”System.Web.UI.WebControls” assembly=”System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35″ />
</controls>
The web.config files are generally found here: C:\Inetpub\wwwroot\wss\VirtualDirectories\[PORT NUMBER]. If the controls tag is already present in the pages tag, then put the <add tagPrefix… tag in the existing controls tag.