you should have yum configuration in /etc. There should be yum configuration file and yum.repos.d directory where you can save the repositories that you want to add.
This is my rpmforge repository enabled in my rhel4 system.
# Name: RPMforge RPM Repository for Red Hat Enterprise 4 - dag
# URL: http://rpmforge.net/
name = Red Hat Enterprise $releasever - RPMforge.net - dag
#baseurl = http://apt.sw.be/redhat/el4/en/$basearch/dag
mirrorlist = http://apt.sw.be/redhat/el4/en/mirrors-rpmforge
#mirrorlist = file:///etc/yum.repos.d/mirrors-rpmforge
enabled = 1
protect = 0
gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY-rpmforge-dag
gpgcheck = 1
Create a file rpmforge.repo in the /etc/yum.repos.d directory and add the contents in the file.