I saw it in this blog post, but that doesn’t actually say how to “enable” it. And it seems that by default it isn’t enabled.
I know it’s an extension method, as defined here: http://msdn.microsoft.com/en-us/library/hh835786(v=vs.108).aspx but how do I get access to it? If I type Request.CreateErrorResponse then the compiler doesn’t recognize it.
I’m already using System.Net.Http.
Are you still using pre-release or release version? There were a number of extensions that did not appear until just before the release and did not exist in earlier versions of the webapi release. I am unsure if this was one of them, but it may be what is causing your problem.