I haven’t put a lot of work into this, because it seems like it’s not used much, but if you select the Show Existing WP Comments option, existing WordPress comments will be displayed if there are any existing WordPress comments. The form to post new comments should also be displayed. If it isn’t being displayed, the issue could be with your theme.
If there are no existing WordPress comments, the WordPress comment form will not be displayed. The plugin has always behaved this way. It wouldn’t be difficult for me to add a filter to the logic there, so that you could override this behaviour.
If the Load Comments With Ajax option is selected, it’s not currently possible to get existing WordPress comments or the WordPress comment form to display.