define( 'WP_ADMIN', TRUE );
define( 'WP_NETWORK_ADMIN', TRUE ); // Need for Multisite
define( 'WP_USER_ADMIN', TRUE );

require_once('../wp-load.php');
require_once( '../wp-admin/includes/admin.php' );
require_once( '../wp-admin/includes/plugin.php' );

activate_plugin( 'PATH_TO_THE_PLUGIN' );