For Rice RHN-controlled RHEL5 hosts, the easiest way to install the Zabbix agent is through the EPEL RPM.
RHN has zabbix_agentd Version 1.4.7 which has many problems with Zabbix 1.8.3 on monitor.
You should install instead zabbix_agentd 1.8.x
If you haven't already added EPEL (Extra Packages for Enterprise Linux) you should start with:
yum -y install epel-release
This installs files in /etc/yum.repos.d/ and enables the epel repository by default.
Next, install the zabbix-agent RPM:
yum -y install zabbix-agent
On some servers, leaving EPEL enabled may not be in keeping with best practices to keep as close to the distribution rpm set as possible, so you can set the EPEL repository to be disabled by default.
perl -npi -e 's/enabled=1/enabled=0/' /etc/yum.repos.d/epel.repo