Good catch - these are outdated instructions from back when Doorkeeper didn't have this field. No need to send an email as well, I'll update the instructions on the blog post.
Sidenote: Doorkeeper.jp has improved a great deal in the last few months. Check it out for a great event management system.
We've started this service in Japan, but have always had the international market in mind. If you're an organizer who holds regular events, we'd love to help you out. See http://www.doorkeeperhq.com/ or shoot me an email at paul@mobalean.com.
Sidenote: Doorkeeper.jp has improved a great deal in the last few months. Check it out for a great event management system.