To replace syslogd with syslog-ng.
make & make install
After installation completely , edit /etc/rc.conf to let the system use syslog-ng instead of syslogd.
add this line on /etc/rc.conf
To kill process syslogd process.
kill `cat /var/run/syslog.pid`
copy syslog-ng.conf.sample to syslog-ng.conf
cp /usr/local/etc/syslog-ng/syslog-ng.conf.sample /usr/local/etc/syslog-ng/syslog-ng.conf
In case you don't want to change sylogn-ng configuration. you can start syslog-ng process by running this command