Forked from https://github.com/google/protobuf. The oringal C# project uses some new APIs of .NET 4 which do not supported by Unity/Mono. I've made some changes to the code to make it compatiable with Unity/Mono.
Open the project file (./csharp/src/Google.Protobuf/Google.Protobuf.Unity.csproj) in MonoDevelope (or Visual Studio, Xamarin Studio) and build.
Copy Google.Protobuf.Unity.dll to the Assets folder of Unity project and enjoy it!