How do I manually install extensions on Chrome for OS X?

First, when you look at your currently installed extensions (even if there aren't any), click on the "Developer Mode" link. This should show you a button labeled "Load unpacked extension..." -- that's the main way to load your own extensions.

You can also review your published extensions on the developer dashboard. On the page for browsing extensions, look for the link that says "Publish your extensions" on the left side. That should take you to a developer dashboard, you'll probably be prompted to log in to your google account first.

Google provides tons of help building Chrome extensions. Try the Getting Started page.

The direct answer to your question is that Chrome extensions are stored in: /Users/username/Library/Application Support/Google/Chrome/Default/Extensions but you probably shouldn't try to manipulate things directly there -- use the tools within the browser.

In OS X Lion the Chrome extension folder is here:

/Users/<username>/Library/Application Support/Google/Chrome/Default

If you cannot see Library folder, you may open Finder, choose Go > Go to Folder..., then type ~/Library and hit Enter. After that, you can browse Chrome folder.