How to install extensions : Extensions - Google Chrome Help
1. Visit the store at http://chrome.google.com/webstore. You can also reach it by clicking the icon in the “Apps” section of the New Tab page.
2. Browse or search for the extension you’d like to install.
3. On the extension’s details page, click the Install button.
4. A dialog appears, letting you know about the types of data that the extension will be able to access. Click Install in the dialog to grant the extension access to the data described.
The extension installs and loads automatically, so you can start using it right away.