Dewey is a Chrome App to better organize and tag your bookmarks. It's an app, not an extension. So you got to launch it from the app page. Type chrome://apps in the address bar if you lost access to the app page, you'll find Dewey there.
If you're using both Delicious and Chrome, but not using the social side of Delicious, then you can say bye bye to Delicious and benefits from tags right in Chrome bookmarks. As they synchronize, it works the same (IMHO, better).
And if you've been sorting your Chrome bookmarks into folders, Dewey retains folder names/structure as tags.
One less app!