Looking again, it only offers to import a Google Reader XML export file, not any OPML, and I was uploading an export from The Old Reader, so it shouldn't have worked anyway.
I just fixed it in the code, it will work when I'll deploy the new version.
Cool. I manually subscribed to all my feeds -- slow day at work, and it let me rename things -- but this will let me suggest CommaFeed to friends and family.