How can I use the async keywords in a project targeting.net 4.0

You want the Microsoft.Bcl.Async package. That's a properly released, non-CTP package that is stable. This also requires VS2012 since an updated compiler is needed to understand async and await.

In theory one could use older tools from VS2010 along with the older CTP library, I'd strongly recommend that you don't - it has bugs and the installation itself is a hit-or-miss.

I have written a .NET 4.5 plugin using async for a .NET 4.0 application, and to my surprise this actually worked!

I think this worked because I have .NET 4.5 installed, which replaced the .NET 4 runtime with an updated one, which is used both for .NET 4.0 and .NET 4.5. Then my plugin was loaded with reflection using Assembly.Load(...) or similar. I tried both async/await and Environment.CurrentManagedThreadId (a .NET 4.5 property), and both worked.