To allow visitors to comment, you need to do three things:
In the options for the `comment_form` plugin, enable the options to allow comments on whatever items you want and make sure that "Only members can comment" is disabled.
You must have "Allow comments" enabled for each item you want.
And lastly, your theme must have comments enabled as well.
If you still find that it's not working, it would be helpful to us if you provided a link to your site.