Enable syslog-ng to accept logging from the network /etc/syslog/syslog-ng.conf

source src{
    udp(ip("192.168.1.10") port(514));
}

Substitute the IP address '192.168.1.10' for the IP address of your syslog-ng server. Reload syslogn-ng

# /etc/init.d/syslog-ng reload

Configure DD-WRT to send syslog messages to your syslog-ng server

# nvram set syslogd_rem_ip=192.168.1.10
# nvram set syslogd_enable=1
# nvram commit
# reboot

To enable logs from the firewall you may want to use the following configuration.

# nvram set log_level=2
# nvram set log_enable=1
# nvram set log_rejected=1
# nvram set log_dropped=1
# nvram set log_accepted=0
# nvram commit
# reboot

Logging packets that hit your firewall may overwhelm you with useless information. Once DD-WRT reboots, /var/log/messages on your syslog-ng box should be collecting syslog messages from DD-WRT.