The error is correct.
VS2010 with the Async CTP is not stable and certainly not recommended for production use. Please note that it is not easy to even create this development environment anymore, since the Async CTP is blocked by a number of Windows Updates.
But even if you use VS2012 (i.e., with Microsoft.Bcl.Async), this still won't work because async
behavior is undefined on ASP.NET 4.
To use async
on MVC, you must use ASP.NET 4.5 (or higher), and this means using VS2012.