It sounds like from your error that your User model does not implement UserInterface (the default User Model that comes with Laravel does this properly, so I'm guessing you made a custom one)
http://github.com/laravel/laravel/blob/master/app/models/User.php
Make sure that the User model looks like this:
use Illuminate\Auth\UserInterface;
use Illuminate\Auth\Reminders\RemindableInterface;
class User extends Eloquent implements UserInterface, RemindableInterface {
/**
* Get the unique identifier for the user.
*
* @return mixed
*/
public function getAuthIdentifier()
{
return $this->getKey();
}
/**
* Get the password for the user.
*
* @return string
*/
public function getAuthPassword()
{
return $this->password;
}
/**
* Get the e-mail address where password reminders are sent.
*
* @return string
*/
public function getReminderEmail()
{
return $this->email;
}
// Add your other methods here
}