There are several reason why proftpd might not start.
1) Make sure that it's installed. (RedHat only) You should be able to run
rpm -q proftpd
on RedHat and it should show you the current version. If it says that proftpd is not installed, then you must install it.
cd /usr/local/directadmin/scripts ./proftpd.sh
Watch for errors.
One common problem is another conflicting ftp daemon. Remove the other ftp daemon
rpm -e --nodeps other-ftp
and then run ./proftpd.sh again.
2) Ensure IP's are setup correctly a) make sure that the server ip is setup with the hostname in the /etc/hosts file. b) make sure that the server ip is *not* in the /etc/proftpd.vhosts.conf as a VirtualHost
Once you've got it setup and installed restart it by running:
Redhat:
service proftpd restart
FreeBSD:
/usr/local/etc/rc.d/proftpd restart