That's a bug caused by no books currently in iTunes, which will be fixed.
If you add a book directly to iTunes before running calibre, I think you can get around it for now.
Try this:
- In calibre, export a single epub to your desktop using Save to disk. It doesn't matter what book, any epub.
- Open the exported folder to find the epub
- Drag the epub into your iTunes Library window.
- In iTune's left panel, under LIBRARY, you should now see a 'Books' entry with your added book.
- Start calibre again, you should be OK.
G