Hi Briohny,
Have you tried downloading and updating the site files from a different PC/Mac?
I would try Download userforms-0.1 archive from ss and Install using my steps above. This would at least rule out some sort of common caching problem that we both might have. (If it doesn't work).
Failing that, do a code search in your directory for "EditableMultipleOptionField" and report back on what and where you find it.
Other than that, this is looking more and more like some sort of SS caching issue. But as I say above, I really can't pinpoint the solution for me except for what I posted.
Cheers
Aaron