Admin routing, won't go to posts/admin_index

I have admin routing enabled. How can I set routing, to make go to posts/admin_index?

I've got this:

Router::connect('/', array('controller' => 'posts', 'action' => 'index'));

But it doesn't seem to work. I get this error (when going to

Missing Controller

Error: Controller could not be found.

Error: Create the class Controller below in file: app/controllers/controller.php

class Controller extends AppController {

    var $name = '';