The likely cause is you are running a newer version of Apache than the script was developed under, or the Apache config has been tweaked, so that global variables the script expects aren't available, or use a different name.  There are also some known problems with register globals being turned off.

Adding these lines near the top of the directory file *should* fix either of these causes.

if ($_POST['submit_add']){$show="add";}
if (empty($PATH_INFO)){$PATH_INFO = $_SERVER['PATH_INFO'];}
if (empty($PATH_INFO)){$PATH_INFO = $_SERVER["ORIG_PATH_INFO"];}
if (empty($PATH_INFO)){$PATH_INFO = $HTTP_SERVER_VARS['PATH_INFO'];}
if (empty($PATH_INFO)){$PATH_INFO = $_SERVER["REQUEST_URI"];}

Keep in mind, the directory file is actually a PHP file, it's just lacking the .php file extension. If you need to use an FTP program to download/upload the file for editing, be sure to use ASCII/plain text transfer method.