@Babar: Form the release notes, you'll see that it requires .NET Framework 4.5 and the PI AF Client 2013 Beta.
You must use Visual Studio 2012 in order to utilize .NET Framework 4.5, the 2010 version is not capable of targeting this framework. This is for that reasons and other ones that examples are built with 2012.
What Mathieu said is true-- for the /server/ side.
On the client side, you should be able to build a client in any environment that supports HTTP, including VS2010/.NET 4. The examples we've released to date make use of the HttpClient in the System.Net.Http namespace, introduced in .NET 4.5. HttpClient offers an improved REST development experience over the older WebClient, offering convenient mapping of common HTTP verbs like Get, Put, and Post, that you'd use for REST. But the older WebClient should certainly be usable.