I actually solved this one by paying close attention to the error message I
was getting, which indicated that the Unfiled Notes could not be found. Then
I noticed that I could not open the Unfiled Notes from within OneNote - that
the file was from a later version.
Several weeks ago, I installed Office 2010 Professional Beta (mostly to look
at OneNote), but I removed it when I saw that Office 2010 could not coexist
with Office 2007. Since the computer is used primarily by my family for
school-related work (college down to third grade), I needed to keep it
working like everyone else expects. So I bought OneNote 2007, since it will
be a long time before I'm likely to go to Office 2010. Anyway, OneNote 2010
put an Unfiled Notes file, which was not removed at the uninstallation.
After I deleted this offending file, the Send to OneNote 2007 function works
like I expected it to from everything I tested it in - IE, Outlook, Word,
Adobe Reader....
So BE CAREFUL IF YOU INSTALL OFFICE 2010 AND THEN REMOVE IT!! The
uninstallation IS NOT COMPLETE.
Now, hopefully, I can take full advantage of OneNote 2007.