On the frontend (as root or using sudo) install the package using yum install:

$ sudo yum install postgresql-libs

Sometimes you need to enable a particular repo to find the right package:

$ sudo yum install --enablerepo base,update install postgresql-devel # or
$ sudo yum install --enablerepo epel postgresql-devel

RPM will be downloaded to /var/cache/yum/{REPOSITORY}/packages/. Copy the RPM to /export/rocks/install/contrib/6.1.1/x86_64/RPMS/

Add a <package> entry for the RPM to the XML file /export/rocks/install/site-profiles/6.1.1/nodes/extend-compute.xml. Note that this entry does not contain version numbers.

We then need to recreate the distro:

$ cd /export/rocks/install
$ rocks create distro

You then reinstall (and restart) all the compute nodes. See this wiki entry for more details on how to do that.