I've already bought the upgrade and have added a lot of CSS code.
Which blog exactly are you referring to? The blog linked to your username is devoid of any blogger created content ie. empty and displays no evidence of CSS editing at all. Please post an active link starting with http:// to the blog you have been CSS editing.
Also be mindful of what raincoaster correctly stated. Disqus commenting systems cannot be embedded into free hosted WordPress.com blogs - period. That's true whether or not they have a premium theme because we cannot edit either themes or templates.
The custom design upgrade and CSS editing cannot be used to embed Disqus commenting systems or anything else into a free hosted WordPress.com blog. There is no FTP access to free hosted WordPress.com blogs and we cannot intsall individual plugs inot them. Those found on the internet are for WordPress.ORG installs which run on different software.
Please read this section of the support documentation Custom design > Frequently Asked Questions