uninitialized constant LoadUserData::Fixtures

I'm trying to follow the example on page 273 of Dave and David's book,
and I cannot get my yaml stuff loaded. I've tried several ways,
including both singular and plural. It appears the book has a typo
here because use of the plural in the migration command in the middle
of the page gives you a plural class name LoadUsersData::Fixtures,
which contradicts the singular one displayed below on the same page.
I also got an error at one point about the ___FILE___ or __FILE__
symbol (I still haven't figured out if that's two or three
underscores, but both fail for me anyway...)

I've also noticed that "rails data migration" in google, nor anything
similar I could think of, yields me examples of a load of data from
YAML. Please let me know of such a simple example page. If I can get
the thing going simply, I'm sure I can make progress from there.
Thank you for any feedback.