You need register another directory/namespace such as the documentation says
<?php
// For Namespaces
$loader->registerNamespaces(
array(
//...
"MyLib" => "../app/library",
"MyPlugin" => "../app/plugins",
)
)->register();
// For directories
$loader->registerDirs(
array(
"../app/library/",
"../app/plugins/"
)
)->register();