I had the same exact problem. Been scratching my head all day! It seemed like it just came out of the blue, since i dont remember doing any updates of VS2010 or any of its add-ins.
But i found out that the problem was due to the old version of NuGet was still lying around.
Going to this folder:
"C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\Extensions\Microsoft Corporation\NuGet Package Manager"
And removing all other folders than the one with the newest version number (1.5.20902.9026 in my case) did the trick.
When i started VS2010, i got the new "Manage NuGet packages" menuitem, which has replaced the old "Add library reference" from the old version.
I also did some reinstallation of the plugin, and even of VS2010, but in the end i think it was enough just to remove the old-version from the "NuGet Package Manager" folder as mentioned above.
Hope this saves you some time :-)