WOW !! What job.
Ok, here is the solution. It appears that the Visual Studio pluggin does NOT update when you update Xamarin Studio on your window. HOWEVER, when you go to your Mac build server and upgrade Xamarin to the Alpha version, then open the Xamarin project in Visual Studio, it will suggest that the build server and Pluggin be different versions and allow you to upgrade. When you update, after closing the visual studio, then open the visual studio again, VOILA !!! The System.Net.Http link is now available in the Assemblies section when adding a link.
So, the solution is to upgrade your MAC version of Xamarin Studio to Alpha, and then Visual Studio will prompt you to upgrade. Upgrade it and you are golden!
It seems like there should be an easier way, but it worked.
source share