-your .htaccess file is configure incorrectly or doesn't exist.

Copy and paste the code below into your .htaccess file

DirectoryIndex index.php

php_flag register_globals off
php_flag magic_quotes_gpc off

FileETag none
ServerSignature Off

Options All -Indexes

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^shorten/(.*)$ shorten.php?longurl=$1 [L]
RewriteRule ^([0-9a-zA-Z]{1,6})$ redirect.php?url=$1 [L]
</IfModule>

For details comment, visit http://zeetmarket.com/downloads/Papu-Url-Shortener-Script