You can just copy/paste the following lines:
esxcfg-firewall -e ntpClient
echo “server clock.redhat.com” >> /etc/ntp.conf
echo “clock.redhat.com” > /etc/ntp/step-tickers
chkconfig ntpd on
Just replace clock.redhat.com with your NTP server. If you want more than one time server, just echo the lines more. Use >> for the step-tickers line to append rather than overwrite.