CiviCRM extensions add extra functionality to your PowerBase database. Most extensions are disabled by default. If you would like to get the extra functionality you have to enable them.
The admin user of your site has the permissions necessary to enable extensions, however, we ask that you please email support to let us know if you are enabling a new extension so we can help you use it most effectively.
Go to the CiviCRM manage extensions page
Click the Administer -> Systems Settings -> Manage Extensions
[Note: if you don't see the Manage Extensions menu here, try in the "Customize Data and Screens" menu.]
Find the Extension you want to enable
When you find your extension in the list, click the "Install" link to the right of the extension name. In this screen shot I am enabling the Last Assessment extension.