You can install the Sourcetab extension through the Downloads page. Click on the link to your browser's extension store and install from there. If you would like to try out Sourcetab before installing, you can view the Demo page.
Once installed, make sure the extension is enabled. If everything is set up correctly, clicking the new tab button should open Sourcetab.
You can use this documentation to learn more about the features built into Sourcetab.