While RedHat (and its derivatives like CentOS) have lots of packages that you can install, not everything is there. Repoforge seems like the other big source of packages. Here is how to get your CentOS to know that repository:
AdditionalResources/Repositories/RPMForge - CentOS Wiki
Download the rpmforge-release package. Choose one of the two links below, selecting to match your host's architecture. If you are unsure of which one to use you can check your architecture with the command uname -i
x86_64 http://packages.sw.be/rpmforge-release/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm (this redirects alot but ends up at http://apt.sw.be/redhat/el6/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm)
The preferred rpmforge-release package to retrieve and to install in order to enable that repository is one of the two listed above.
Install DAG's GPG keysudo rpm --import http://apt.sw.be/RPM-GPG-KEY.dag.txt
If you get an error message like the following the key has already been imported:error: http://apt.sw.be/RPM-GPG-KEY.dag.txt: key 1 import failed.
Verify the package you have downloadedsudo rpm -K rpmforge-release-0.5.2-2.el6.rf.*.rpm
Security warning: The rpmforge-release package imports GPG keys into your RPM database. As long as you have verified the md5sum of the key injection package, and trust Dag, et al., then it should be as safe as your trust of them extends.
Install the packagesudo rpm -i rpmforge-release-0.5.2-2.el6.rf.*.rpm
This will add a yum repository config file and import the appropriate GPG keys.
Then try to install something like thissudo yum install pdsh