auto logout

I have just enabled autologout module. I have two questions.

1. If I enable Enable watchdog auto-logout logging , where will I see the user loggout messages.

2. I saw following method in autologout module

function _autologout_debug($s) {

if (defined("AUTOLOGOUT_DEBUG") && AUTOLOGOUT_DEBUG) {
error_log($s);
}
}

How do I set AUTOLOGUT_DEBUG to be true to see log trace? Also, where do I see output from error_log?

Thanks for your help.
Chinni