cd /usr/local/directadmin/custombuild
./build update
./build set proftpd yes
mkdir -p custom/proftpd
wget -O custom/proftpd/configure.proftpd http://files.directadmin.com/services/all/sftp/configure.proftpd.sftp
chmod 755 custom/proftpd/configure.proftpd
./build proftpd

2) Backup old configs, and grab new ones:
cd /etc
cp proftpd.conf proftpd.conf.backup
wget -O proftpd.conf http://files.directadmin.com/services/all/sftp/proftpd.conf
wget -O proftpd.sftp.conf http://files.directadmin.com/services/all/sftp/proftpd.sftp.conf
perl -pi -e 's/\|SERVER_IP\|/1.2.3.4/' proftpd.conf
perl -pi -e 's/\|SERVER_IP\|/1.2.3.4/' proftpd.sftp.conf