Instance a Default Zend_Db_Table_Adapter

//Config Example
$config = new Zend_Config(
        array(
            'database' => array(
                'adapter' => 'mysqli',
                'params'  => array(
                    'host'     => 'localhost',
                    'dbname'   => 'test',
                    'username' => 'username',
                    'password' => 'password',
                )
            )
        ));

//Do de Factory
$db = Zend_Db::factory($config->database);
//Set as default adapter
Zend_Db_Table_Abstract::setDefaultAdapter($db);