Change into your Plugin directory, and checkout the git repo:

cd Plugin
git clone git://github.com/lorenzo/MongoCake.git
cd MongoCake
git submodule update --init --recursive

You first need to activate the plugin in CakePHP after placing it in the correct folder:

CakePlugin::load('MongoCake', array('bootstrap' => true));

Use this MongoCake plugin like any other datasource, with its own configuration options:

// Within Config/database.php
public $default = array(
	'datasource' => 'MongoCake.CakeMongoSource',
	'server' => 'localhost', // Optional
	'database' => 'mydatabase', // Database to use
);

Ensure that your models extend the CakeDocument class.

class User extends CakeDocument {
}