The information in the Flickr Help for Favorites is for the current design and states:
We're only providing support for the normal site during the preview. Answers that you receive here (from staff & members) will relate to the current design not the preview. If you're having trouble with something that isn't covered in the FAQs you can opt out. If you have questions or feedback please let us know in the New Photo Page Preview Group.
So, has anyone figured out how to favorite a photo using the new-fangled preview page?