You probably haven't installed and/or enabled the PHP MySQL extension ([it's a requirement][1]). You can check by [creating a `phpinfo()` page][2] and searching for 'mysql'. If it's not there, you need to install and enable it. You don't say what server you're using, but if it's Ubuntu or a similar Debian-based variant, you can do this:

    $ sudo apt-get install php5-mysql
    $ sudo /etc/init.d/apache2 restart

If you're on another platform, the steps will be a bit different, but you get the idea.


  [1]: http://drupal.org/requirements/php
  [2]: http://kb.mediatemple.net/questions/764/How+can+I+create+a+phpinfo.php+page?#gs