The instructions you read were intended for our other plugin that we have integrated into the WordPress.COM platform and cannot be accessed by users whose blogs are hosted on WordPress.ORG (like yours).
Because of that, I would recommend you to just install & activate this plugin instead.
It's also worth noting that this plugin only uses thumbnails from your own blog posts (and none others), so if your images on your blog don't interfere with any copyrights, you should have nothing to worry about.
But if you would like to use copyright cleared images from around the web in your future blog posts, as well as quality, related articles from fellow bloggers, than the 'Editorial Assistant by Zemanta' is the plugin you are looking for. Here's the link:
And here's the link to our demo page, if you would like to see it in action first, before installing it: http://www.zemanta.com/demo/
Let me know what you think or if you have any more questions what-so-ever — I'll gladly help!
Take care & have a nice day!