ExportRenderer missing assembly

Dear all,

I am following this tutorial on Xamarin.Auth, and I am missing something. When compiling I get the following error:

Error CS0246: The type or namespace name `ExportRenderer' could not be found. Are you missing an assembly reference? (CS0246) (Test.iOS)

I think I've followed the instructions step by step, and right now I am focusing only on iOS, so no Xamarin.Auth is installed on the Test.Droid project. The code I've written is this, basically a copy/paste from the tutorial:

using System;
using Xamarin.Forms.Platform.iOS;
using Xamarin.Auth;

[assembly: ExportRenderer (typeof (LoginPage), typeof (LoginPageRenderer))]
namespace Test.iOS
{
    public class LoginPageRenderer : PageRenderer
    {
        public override void ViewDidAppear (bool animated)
        {
            base.ViewDidAppear (animated);
            // ...
            PresentViewController (auth.GetUI (), true, null);
        }
    }
}

What I've found googling around is this post, but after re-installing Xamarin.Forms (version 1.3.5.6335) on both Forms and iOS project nothing changed.

What am I missing?

Thanks!