ARGH!
I found it… I didn’t have an extra package, called Microsoft.Owin.Host.SystemWeb
Once i searched and installed this, it worked.
Now – i am not sure if i just missed everything, though found NO reference to such a library or package when going through various tutorials. It also didn’t get installed when i installed all this Identity framework… Not sure if it were just me..
EDIT
Although it’s in the Microsoft.Owin.Host.SystemWeb assembly it is an extension method in the System.Web namespace, so you need to have the reference to the former, and be using the latter.