Monitoring Linux Host

Hi,

I am a newbie in Nagios, i was able to successfully installed Nagios on my monitoring server including the plugins and nrpe addons to monitor our linux servers. Now, i am having problem with configuring the nrpe. for details, here are my configurations.

Monitoring Hosts:

/usr/local/nagios/etc/objects/commands.cfg

define command{
command_name check_nrpe
command_nrpe $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG!$

define command{
command_name check_users
command_line $USER1$/check_users -w $ARG1$ -c $ARG2$
}

/usr/local/nagios/etc/objects/testlinux.cfg

define host{
use testlinux
host_name database_server
alias Centos
address <ip_address of remote linux>
}

define service{
use generic-service
host_name database_server
service_description Current Users
check_command check_nrpe!check_users !20!50
}

Remote Linux

/usr/local/nagios/etc/nrpe.cfg

command[check_users]=/usr/local/nagios/libexec/check_users -w 5 -c 10

THIS IS THE ERROR I AM GETTING FROM THE WEB GUI

(Return code of 127 is out of bounds - plugin may be missing)

Can someone help me on how to step by step configure it? I follow the instructions on the geekstuff.com but it seems i can only monitor the local host.

Thanks.