Master Page

1. Add the following tag just below the form tag of master page


 <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePartialRendering="true" />


Web.Config

1. Add safe control


 <SafeControl Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" Namespace="System.Web.UI" TypeName="*" Safe="True" />


2. Add the http handler inside <httpHandlers> tag of web.config


<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions,	Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory,	System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions,	Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false" />


3. Add the HttpModule inside <httpModules> tag of web.config


 <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral,PublicKeyToken=31bf3856ad364e35" />


4. Add the assembly inside <assembly> tag of web.config


 <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />


5. Add controls Tag inside <pages> tag of web.config

<controls>
	<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral,		PublicKeyToken=31bf3856ad364e35" />

</controls>



If you are adding <system.webServer> tag inside <configuration> like following then you could avoid step 2 and 3


<system.webServer>

<validation validateIntegratedModeConfiguration="false" />

<modules>

<add name="ScriptModule" preCondition="integratedMode" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />

</modules>

<handlers>

<remove name="WebServiceHandlerFactory-Integrated" />

<add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0,Culture=neutral,PublicKeyToken=31bf3856ad364e35" />

<add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral,PublicKeyToken=31bf3856ad364e35" />

<add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral,PublicKeyToken=31bf3856ad364e35" />

</handlers>

</system.webServer>