public class MainActivity : AppCompatActivity
{

    protected override void OnCreate(Bundle savedInstanceState)
    {
         base.OnCreate(savedInstanceState);
         SetContentView(Resource.Layout.activity_main);

         //Botones y cosas
         button.Click += delegate {

            Intent i = new Intent(this, typeof(SegundaActivity));
            StartActivty(i); //<-- Android.Content.ActivityNotFoundException:
                             //    unable to find explicit activity class

            }
     }

}

public class SegundaActivity : AppCompatActivity{

     protected override void OnCreate(Bundle savedInstanceState)
     {

        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.activity_segunda);

      }
}