I'm having the same problem that others have reported:
WordPress 3.5 add new media is not working, so it's impossible now to add a new file when I want to post a new post or page.
From other threads I've found this solution:
Add: define('CONCATENATE_SCRIPTS', false ); in your wp-config.php file just before require_once(ABSPATH . 'wp-settings.php');
I'm new at this and don't know how to locate that file. Any help would be appreciated.