LOGIN_PAGE is defined in the auth_constants hook, and it's a pre_system hook. In config/config you should set enable_hooks to TRUE.
There is no magic here. Either auth_constants hook runs because it's set up correctly or not.
Note installation step 5:
5) Copy auth_constants.php and auth_sess_check.php from community_auth/hooks to CodeIgniter's application/hooks directory.
If these files are in your application/hooks directory, you've enabled hooks in config/config, and put the following in config/hooks.php, then it should work:
Code:
$hook['pre_system'] = array(
'function' => 'auth_constants',
'filename' => 'auth_constants.php',
'filepath' => 'hooks'
);
$hook['post_system'] = array(
'function' => 'auth_sess_check',
'filename' => 'auth_sess_check.php',
'filepath' => 'hooks'
);