Before the RC release of MVC4/WebAPI JsonArray and JsonObject lived in System.Runtime.Serialization.  Once you upgrade your project these options disappear.

The fix is pretty simple.  In the latest RC release of MVC4/WebApi Microsoft switched the JSON handler to JSON.NET

The project should have a reference to Newtonsoft.Json (if it does not, you might need to load the NuGet package.

Once this is complete add a using statement

using Newtonsoft.Json.Linq

After that you can now use:

• JArray in place of JsonArray
• JObject in place of JsonObject
• so on and so forth.