If you're at the folder level, it's on the Home tab. If you're in an email, it's on the message tab. Look for the purple icon (it no longer says "Send to"...it just has a blue arrow pointing towards the purple OneNote icon).
Note that unlike in 2007, where it immediately created a new tab among your Unfiled Notes, in 2010 you first get a dialogue box that makes you pre-select where in your OneNote folders you want it go, saving you the trouble of having to file it later
If you're at the folder level, it's on the Home tab. If you're in an email, it's on the message tab. Look for the purple icon (it no longer says "Send to"...it just has a blue arrow pointing towards the purple OneNote icon).
Note that unlike in 2007, where it immediately created a new tab among your Unfiled Notes, in 2010 you first get a dialogue box that makes you pre-select where in your OneNote folders you want it go, saving you the trouble of having to file it later