Right-click on a folder and choose "Open as Notebook in OneNote" was a little used power feature in OneNote 2007, that was removed in 2010 because many people were confused by it. Many people just didn't know how to use it and we improved our open experience in OneNote to people wouldn't need to do this. That said, if you _really_ missed this you can add it back via registry entry, just add following:
Windows Registry Editor Version 5.00
@="Open as Notebook in OneNote"
(or whatever you've for your location of OneNote)
This should be it and you are done.