I was asked how to do this and wrote up some "down and dirty" notes on what I found works well.
Assuming you have a toolbar based addin, here's the steps to get it working for OneNote 2010 (and only on OneNote 2010):
Find and replace “2007” strings in schema namespaces to “2010”
Delete the ON 12 COM reference
Add the ON 14 COM reference (set NOPIA if using VS2010 - http://blogs.msdn.com/b/descapa/archive/2010/04/27/onenote-2010-and-visual-studio-2010-compatibility-issues.aspx)
---these next 2 steps only apply if you created an addin that uses a toolbar icon to be invoked---
Change your GUID to be unique so that you don’t stomp on 12 addins
And lastly, change the OnEvent handler to this:
bool OnEvent([In] OneNote.IOneNoteEvents evt, [In] String strParameter);
Change the registry settings for the toolbar info from the 12.0 branch
Change the COM Import GUID [ComImport, Guid("C9590FA7-2132-47fb-9A78-AF0BF19AF4E6")]
We're working on getting the OneNote 2010 documentation finished, but if you have older code you want to see working with OneNote 2010, this may get you going for now.
Questions, comments, concerns and criticisms always welcome,