Recently I have created my own package, which is handling the authorization (I don't want to use Fuel's auth packages, because of my database structure).
So I am initializing it in the config.php:
'packages' => array( 'parser', 'myauth', ),
And whether I am trying to access any of the class function:
If MyAuth is your package, then \MyAuth is the namespace and thus an undefined constant for PHP since it's assuming a class name when you do $auth = \MyAuth.
You want ensure following the singleton approach for your auth package, so you need at first a class \Myauth\Auth with its static _init function to set the singleton instance. Then, you may get access to your auth instance via \Myauth\Auth::instance() or \Myauth\Auth::$instance if the property is public.
Take a look at other, simpler classes like \Fuel\Core\Theme which also handle singleton instances so you'll get what I'm talking about ;) The auth package might not be a good example for understanding singleton in Fuel since it uses some crazily awesome driver-loaded class layout, and it anyway is core-namespaced which is why you an use e.g. \Auth::get_user() instead of \Auth\Auth::get_user() (here, the first \Auth is the namespace, the second \Auth the class being accessed).
Hope that was to be understood, otherwise ask here or (to get quicker responses) on #fuelphp :)
PS: Class and namespace names should following naming convention which is ucwords, that's why I was using Myauth over MyAuth in the example.