Search The Web is a simple widget that allows you to quickly perform searches from your Dashboard. Search The Web is able to work with Google, Yahoo, Bing, Wikipedia, Facebook and Twitter.
The Search The Web widget will install via its built mechanism and all you have to do is double click the .wdgt file resulted after unarchiving the downloaded package. Once installed, your Dashboard will become active and will display the new widget.
To learn more about dealing with the Mac OS X Dashboard you can easily access the Apple support page HERE.