Have you disabled apparmor? Also, do the following and your system should create the missing directories at boot up with the proper permissions.
Edit /etc/rc.local and add the following before the exit line:
chown -R postfix:postfix /var/run/MailScanner
chown -R postfix:postfix /var/lock/subsys/MailScanner
After you've done that, reboot your system and see what happens. If you're still getting errors, please post them here.