If you are using Red Hat Enterprise Linux, and have not registered with the Red Hat Network, you won't be able to install packages using the YUM tool.
But if you add a centos repository to the /etc/yum.repos.d direcotry, you will be able to do so, since they don't differ much.
Just create a centos.repo file in the directory, and copy paste the below lines, run "yum update" command, and install what ever you wish using yum tool.
[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
mirrorlist=http://mirror.centos.org/centos-5/5.6/os/i386/
baseurl=http://mirror.centos.org/centos-5/5.6/os/i386/
gpgcheck=1
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4
gpgkey=http://mirror.centos.org/centos-5/5.6/os/i386/RPM-GPG-KEY-CentOS-5
protect=1
#released updates
[update]
name=CentOS-$releasever - Updates
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/
mirrorlist=http://mirror.centos.org/centos-5/5.6/updates/i386/
baseurl=http://mirror.centos.org/centos-5/5.6/updates/i386/
gpgcheck=1
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4
gpgkey=http://mirror.centos.org/centos-5/5.6/os/i386/RPM-GPG-KEY-CentOS-5
protect=1
#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=addons
#baseurl=http://mirror.centos.org/centos/$releasever/addons/$basearch/
mirrorlist=http://mirror.centos.org/centos-5/5.6/addons/i386/
baseurl=http://mirror.centos.org/centos-5/5.6/addons/i386/
gpgcheck=1
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4
gpgkey=http://mirror.centos.org/centos-5/5.6/os/i386/RPM-GPG-KEY-CentOS-5
protect=0
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/
mirrorlist=http://mirror.centos.org/centos-5/5.6/extras/i386/
baseurl=http://mirror.centos.org/centos-5/5.6/extras/i386/
gpgcheck=1
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4
gpgkey=http://mirror.centos.org/centos-5/5.6/os/i386/RPM-GPG-KEY-CentOS-5
protect=0
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/
mirrorlist=http://mirror.centos.org/centos-5/5.6/centosplus/i386/
baseurl=http://mirror.centos.org/centos-5/5.6/centosplus/i386/
gpgcheck=1
enabled=0
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4
gpgkey=http://mirror.centos.org/centos-5/5.6/os/i386/RPM-GPG-KEY-CentOS-5
protect=0
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/
mirrorlist=http://mirror.centos.org/centos-5/5.6/contrib/i386/
baseurl=http://mirror.centos.org/centos-5/5.6/contrib/i386/
gpgcheck=1
enabled=0
#gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4
gpgkey=http://mirror.centos.org/centos-5/5.6/os/i386/RPM-GPG-KEY-CentOS-5
protect=0
