To best of my knowledge, there is no a way to do it. (Unless you have a full source code of Tasks :-).) This library is simply not available in .NET 3.5. By the way, the version of C# does not matter at all.
You need to install .NET Framework v.4.0, that's it. It requires XP or later version of Windows. How to do development if you don't have VS 2010? Well, you can use 1) free-of-charge Visual Studio Express 2010 (
http://en.wikipedia.org/wiki/Visual_Studio_Express[
^],
http://www.microsoft.com/express/Windows/[
^]); 2) SharpDevelop (
http://en.wikipedia.org/wiki/SharpDevelop[
^],
http://sharpdevelop.com/[
^]), 3) develop without IDE or create your own IDE — C# compiler is always bundled with .NET Framework (freely re-distributable).
—SA