Get Extensions/Add-ons for Joomla CMS from here : Extensions.Joomla.org
After downloading the zip file ,
- Go to your Administrator Panel.
- Go to Extensions => Install/Uninstall.
- Under the Upload Package File , Click on Choose file and browse to your downloaded zip file.
- After choosing it , click Upload File and Install button to finish installing the Extension.
To confirm your installation, check the Plugins Tab and check if it it in the list .