The GetOwinContext extension method is in the System.Web.Http.Owin dll which needs to be downloaded as a nuget package (The nuget package name is Microsoft.AspNet.WebApi.Owin)

Install-Package Microsoft.AspNet.WebApi.Owin

See msdn here: http://msdn.microsoft.com/en-us/library/system.net.http.owinhttprequestmessageextensions.getowincontext(v=vs.118).aspx

Nuget package here: https://www.nuget.org/packages/Microsoft.AspNet.WebApi.Owin

However, the method is still part of the System.Net.Http namespace, so the using definitions you have should be fine.

EDIT

Okay, to clear up some confusion: If you are using an ApiController (i.e MyController : ApiController) you will require the Microsoft.AspNet.WebApi.Owin package.

If you are using a regular Mvc controller (i.e. MyController : Controller) you will need the Microsoft.Owin.Host.SystemWeb package.

In MVC 5 the pipelines for Api and regular MVC were very different, but often have the same naming conventions. So an extension method in one does not apply to the other. Same for a lot of the action filters etc.