Create a forum for your comments (I called mine Comments). Set the permissions to either public or registered. Don't forget to publish the forum after it is created.
Kunena Discuss Settings:
Allow Custom Topics: No
Topic Owner: (use the id of the admin)
Default Forum Catagory: (use the id of the Comments forum)
Allowed Content Catagories: 0 (all comments go to the comments forum)
You can map each catagory to a different forum if you want too, but I didn't need it for this.
I highly recommend this plugin as comments are not standard in Joomla.