My Access 2007 computer is off helping a friend at the moment, but as I
recall there's an option in the Access Options dialog -- maybe among the
option for the Current Database -- about allowing design changes in
datasheet view (or something like that). (I'm not sure of the exact option,
so if you can't find it, I hope someone else will post to this thread.)
That option should be checked.
You should know, though, that you can also add a new field the traditional
way by switching the table into design view and adding the field there.