Right-click on a folder and choose “Open as Notebook in OneNote” was a little used power feature we had in OneNote 2007 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 being said I wanted to let you know that if you _really_ missed this you can add it back via the registry, just add this:
Windows Registry Editor Version 5.00
@="Open as Notebook in OneNote"
(or whatever you have for your location of OneNote)
This should be it and you are done.