mll Pockethold uses Bazaar, so you can use the Bazaar tab in the admin panel. If you do not have this tab, look in the Extensions tab of the admin panel, and click the checkbox next to Bazaar to turn it on.
From Bazaar, you can search, install, enable and uninstall extensions as needed.
The method samtto mentions is for those who have SSH access and do not use Bazaar.