Create a directory in the base directory of your ~friendica installation called
addon and extract the addon archive files found in the addon repository of ~friendica into that directory. You can also clone the friendica-addon repository into the addon directory instead if creating it and unpack the archives manually using
git clone https://github.com/friendica/friendica-addons addon.
To enable an addon using the .htconfig.php file search for the line containing
$a->config['system']['addon'] (or add it to the file) and add the name of the addon to the list of enabled addons. An example could look like this:
$a->config['system']['addon'] = 'piwik, twitter, statusnet';
You should add the needed configuration variables for the addons according to the documentation of the addons.
If you log into the admin account and select the admin panel from the menu you will find a submenu
plugins with a list of the available addons. From there you can enable or disable any of the listed addons.
If an activated addon offers a configuration interface through the admin panel a link in the side menu for that addon will appear so you can reach the interface.
The Facebook connector does not provide a configuration interface from the admin panel yet, please see the setup guide for installation instructions.