Background: After I updated Xamarin Studio to version 6, my studio suddenly ceased to understand the code: almost everything was underlined in red, all using was written
"Using directive is unnecessary."
I did not understand the studio any code, even new projects Android App. Finding the same problem only here and seeing that there was no intelligible solution, I followed the example of the questioner and made a downgrade by installing Xamarin Studio 5.10.3. But this did not help, Xamarin still does not understand the code.
The problem itself: Xamarin red highlights almost all methods related to android. Hovering on any "using Android." He speaks
"error CS0103: The name 'Android' does not exist in the current context".
He says the same things about Activity, Bundle, SetContentView. I tried to install almost all versions of Xamarin 5.10. Also completely deleted and reinstalled the Android SDK. Nothing helps. Thanks in advance for your help. 
