Ann,
You should give desquinn's suggestion a try as using an FTP address such as this would have been my suggestion as well:
ftp://yourdomain.com/../../var/log/httpd/
This will mean that once you are logged into the FTP account and reside within the home directory of the site admin at /home/siteadminuser/ it will then move up two directories to "/" and then down into /var/log/httpd/ where the log files exist.
If this still doesn't work, please open a support ticket and I will simply make a symlink (shortcut) within the site admin home directory so that the path /home/siteadminuser/var/log/httpd/ will translate to the /var/log/httpd/ directory.