In reply to Justin Lewis.

Had the same problem, and resolved it. In my case, my (self-hosted) WordPress site (using Jetpack) is https:// not http://. I had a few lines in my .htaccess file that FORCE https (to prevent search engines from indexing both):

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

I removed those lines, and presto - Authorization worked as intended.

After I was authorized, I added them back, and the add-on works perfectly. It was apparently only a problem during the authorization step.

Hope that helps!