This line was my headache for several hours
$loader = new \Phalcon\Loader();
/**
* We're a registering a set of directories taken from the configuration file
*/
$loader->registerNamespaces(array(
'MyApp\Controllers' => __DIR__ . '/../controllers/',
'MyApp\Controllers\Admin' => __DIR__ . '/../controllers/admin' //<---- this line!
))->register();
but at last I found solution how good it feels to learn a little more Phalcon