C# – n explanation behind the error “The type or namespace name ‘Script’ does not exist in the namespace ‘System.Web'”

I made an "ASP.NET Empty Web Site" in Visual Studio Express 2010.

On a code file in the App_Code, I have the line "using System.Web.Script.Serialization;"

When I pushed the files to the server, I got the following error:

Exception message: c:\inetpub[….]\Extensions.cs(10): error CS0234:
The type or namespace name 'Script' does not exist in the namespace
'System.Web' (are you missing an assembly reference?)

(The server is running Windows Server 2008 with IIS 7 with ASP.NET 4.0 installed.)

Since this is a web site, and not a web project, I couldn't use the "publish" feature as promoted here: The type or namespace name 'Script' does not exist in the namespace 'System.Web'

Finally, after some digging, I figured out that this namespace is included in the System.Web.Extension assembly, and that I had to manually include it via the web.config:

<compilation debug="true" targetFramework="4.0">
    <assemblies>
        <add assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
    </assemblies>
</compilation>

So, my question: is there a better way to go about this?

Since none of this is needed on my local project, how can I first determine what assemblies I'll need to reference manually, and second, how can I get the information required to do the referencing (i.e., how do I get the PublicKeyToken)?

UPDATE

Because this is a Web Site and not a Web Project, my list of references does not show up until after I add them to the Web.Config manually.

So, to select "Copy Local" to "true" does not work in this instance.