Note: If you are using an older version of QGIS (< 1.0.0), and you haven't yet used the 'Plugin Installer', then you'll probably want to enable it by going to 'Plugins' > 'Plugin Manager' and enabling 'Plugin Installer'. At this point, it's a good idea to update to the latest version of the installer by goin to 'Plugins' > 'Plugin Installer' and reinstalling it.
Windows: extract to 'C:\Documents and Settings\_user_name_\.qgis\python\plugins'
Linux: extract to '/home/_user_name_/.qgis/python/plugins'
Mac: extract to '/Users/_user_name_/.qgis/python/plugins'
where _user_name_ is YOUR user name.