Okay, now I can get the "Upload" button do something.

Okay, when I log off and then back in I can get the "Upload" button to act like it's uploading again.
After thinking about it some I realized I have a couple of custom icons in the stack. From looking at previous comments in this vein, I conclude the reason for the failure to upload is likely these custom icons.

Having custom icons in the resource fork shouldn't prevent the import from working. I've actually had some success importing those. If you want, you can email your stack to me and I'll see what's happening. (ted at tilestack dot com)