I think you might have used “installdir” in the link, as if it were a subdirectory. Kinda like “example.com”. That’s supposed to be a placeholder for the installation directory where you have WP installed, there is no subdirectory called installdir. If you have WP installed in the top level directory, leave it out, just yourdomain.com/wp-mail.php — or if you installed WP in a directory called “wp” then you’d go to yourdomain.com/wp/wp-mail.php.
Oh, and don’t forget to replace yourdomain.com with whatever your domain really is, or it won’t work. ??