To add this site's button, you'll need the latest version of the Google Toolbar. Once you've updated the Toolbar, the custom button should install automatically. If it does not, return to the page where you found the custom button and try installing it again.
Unfortunately, Google Toolbar is only available in Internet Explorer 6.0+ for Windows XP and Vista, and in Mozilla Firefox 2.0+ for Windows 2000, XP and Vista, Mac OSX and Linux.