Cannot reference System.Runtime.Serialization in Unity project

I have a Unity C# project and I want to do some JSON stuff with System.Runtime.Serialization.Json.DataContractJsonSerializer;

I know for a fact I need to include reference to System.Runtime.Serialization and that this works in a non-Unity c# project. But when I add it to my Unity project, Visual Studio does not seem to be able to find the namespace, and the Reference DLL in the References list has a warning sign on it. I also cannot double click the Reference DLL (it says the project is unavailable or is not built, and I cannot build it anyway!)/

 

I just found out Unity uses .net 2.0.0