Running proftpd through xinetd is a bit different. You must stop proftpd if it's running, edit proftpd.conf and use
Then you must add a file named proftpd in /etc/xinetd.d/ directory, containing the following:
flags = REUSE
socket_type = stream
wait = no
user = root
server = /usr/sbin/proftpd
log_on_failure += USERID
disable = no
Just check if the path /usr/sbin/proftpd is correct and restart xinetd.
This is what chkconfig is supposed to do, i.e. create the symlinks to the appropriate rc#.d directories.