The loader class it's usually auto loaded by the Loader class into the bootstrap, anyway DB connect to the database only when you call the get_instance function, and get the info from application/config/conf.db.php file:
$server= "mysql";
$hostname = "localhost";
$username = "root";
$password = "";
$database = "rainframework2";
$path = "";
$hostname = "localhost";
$username = "root";
$password = "";
$database = "rainframework2";
$path = "";