REd Hat 6.4 Nagios/NRPE Adding remote hosts/clients

You need to edit the config files of nagios. There you need to define a host first. This is a basic host:

define host{
	host_name			sample-host
	alias				sample host
	address				192.168.1.20
	parents				switch-core
	check_command			check-host-alive
	check_interval			5
	retry_interval			1
	max_check_attempts		5
	check_period			24x7
	process_perf_data		0
	contact_groups			admins
	notification_interval		30
	notification_period		24x7
	notification_options		d,u,r
	}

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Then you would need to add services and check commands for this host.
I think it is outside of this post to get into that details.

Please read Nagios Object Definitions:
http://nagios.sourceforge.net/docs/3_0/objectdefinitions.html

And, if this is a new deployment, save yourself some cumbersome work with plain text configs and give Nconf a try:
http://www.nconf.org

Which config file should I edit and where...nagios.cfg, cgi.cfg, etc?
i.e. I will try monitoring a file server first...

[root@centos-rhel etc]# pwd
/usr/local/nagios/etc
[root@centos-rhel etc]# ls
cgi.cfg  cgi.cfg~  htpasswd.users  nagios.cfg  nagios.cfg~  nrpe.cfg  objects  resource.cfg  resource.cfg~

hkchris...I have already installed and configured nagios/nrpe.....see previous responses.....Thanks for your input.

Please read “Now adding web-node01 in nagios server so that we could monitor the server's services.” section...

The config files you can use are listed in your /etc/nagios/nagios.cfg
If you haven't touched this, there will be a host.cfg, services.cfg, checkcommands.cfg in objects:
http://nagios.sourceforge.net/docs/3_0/configmain.html

Also, it seems you have installed nagios from the source. Though possible, I would always recommend installing nagios from rpms/repos - since you get updates this way and all the config files will be at the default place for RHEL/centos.

You can find nagios in the epel as well as rpmforge repos. The rpmforge will have newer versions while the epel is considered better because it is run by RH.