In Outlook. try clicking Tools | Trust Center. Under the addins section you
should see OneNote Notes about Outlook Items. Make sure it is not disabled.
If it is, click the Go button near the bottom and enable it.
OK. Try running a detect and repair, and then possibly a re-install. It
sounds like the addin did not get registered for some reason. If you just
ran setup, you can look in your temp folder for the log file to see if there
were any failures.