The file config.php is not writeable by the web server. In order for the install to proceed this needs to be changed
You can do this on a Unix based system by issueing the following command:
chmod a+w config.php
If you do not have command line access to your webserver your FTP client should be able to change file permissions.
Once you have changed the permissions on this file simply refresh this page and the install will proceed normally.
Also, don't forget to remove the writable attribute after running install.php!!