Once you setup Remarkbox on your site, you should be able to moderate comments in place where they appear, regardless of plan.
It seems Remarkbox itself is having trouble reaching your site to verify your ownership. This often happens when people use Remarkbox in a password protected area of a site.
Thanks, I did not realise that would be an issue, I have now lifted the password protection but it is still not working. I also have an issue where I can not scroll up when pointing at the comments, using chrome on osx. The issue is only present on my site and not on this site. Any idea of what can cause that?