With Omeka S 1.0.1, I am adding a new item. I select the Base Resource template, Image class, and give it a name. I switch to the Media tab and select IIIF image under Add media.
The first URL doesn’t look to me like it’s IIIF data of any kind. The second is a IIIF presentation manifest, rather than IIIF image. A IIIF image URL should end in “info.json”. So, for example, one of the images in that manifest is http://lib-sts-16.mit.edu:8080/Cantaloupe-3.2/iiif/2/IAA118513.jp2/info.json which does work (though you get a bad thumbnail due to an error on the remote server’s part).
It turns out that the UI Digital Library doesn’t have an obvious way to get the IIIF API URL from the browsing interface, but the developer gave me the secret incantation.
I’ve now run into a bug (which I’ll file in GitHub). Omeka S tries to download the default.jpg. If it’s too large, the server returns 403, but Omeka S blithely continues on anyway.