There are two ways to use this script:

1. Using the bookmarklet: simply copy the code in the bookmarklet.js file and add it as a new bookmark. Then, whenever you're on a YT video page, click this bookmark to see the download links.
2. Using the Google Chrome extension: go to the extensions page in Chrome, and click on Load unpacked extension..., then choose the folder "Chrome-Extension". Then, whenever you visit a video page on youtube's domain (e.g. http://www.youtube.com/watch?v=_0-SJNnnHFM) the extension will load automatically.

Please, do not use this extension for any thing that would violate any copyrights.