Installing Clustering after initial install (vm on xenserver)
 

Here are the steps I used to get Clustering w/GFS installed on a RHEL 5.5 64bit running in a vm on top of XenServer 5.6. Should work for physical machines also just change the dvd drive from xvdd to sda or whatever yours is.

Make a mount point
#mkdir /mnt/dvd
Mount the dvd (you device path may be different)
#mount /dev/xvdd /mnt/dvd
Create the repo config file. I have repos on the dvd disabled by default so yum does not normally check them and mess up if the dvd is not in.
#vi /etc/yum.repos.d/dvd.repo
[dvd]
mediaid=1170972069.396645*
name=DVD for RHEL5
baseurl=file:///mnt/dvd/Server
enabled=0
gpgcheck=0
[cluster]
mediaid=1170972069.396645*
name=DVD for RHEL5
baseurl=file:///mnt/dvd/Cluster
enabled=0
gpgcheck=0
[cluster-storage]
mediaid=1170972069.396645*
name=DVD for RHEL5
baseurl=file:///mnt/dvd/ClusterStorage
enabled=0
gpgcheck=0

Install Clustering group
#yum groupinstall Clustering --enablerepo=cluster --enablerepo=dvd --enablerepo=cluster-storage
Install Other Clustering stuff
#yum install gfs2-utils.x86_64 sg3_utils-libs.x86_64 kmod-gnbd-xen.x86_64 sg3_utils.x86_64 lvm2-cluster.x86_64 gnbd.x86_64 iscsi-initiator-utils.x86_64 kmod-gfs-xen.x86_64 --enablerepo=cluster --enablerepo=dvd --enablerepo=cluster-storage

Thats it. Pretty straight forward. Can write how to do a basic config if needed