The JPackage project provides it's RPM's through apt, yum, urpmi or up2date using yum repositories. This guide will use the yum method but it is easy to use other methods, there is a guide to setting up other methods on their site.
Yum on CentOS stores its repositories in /etc/yum.repos.d you'll simply need to copy the provided repository there and edit it to your needs:
cd /etc/yum.repos.d wget http://jpackage.org/jpackage.repo
Edit the jpackage.repo file you retrieved and enable the repositories appropriate to your distribution. In the case of CentOS I set enabled=1 on the jpackage-rhel sections.
The JPackage RPMs are digitally signed using a GPG key, import their key into your RPM keyring
rpm --import http://jpackage.org/jpackage.asc
Before being able to use the new repositories you'll need to build a local cache of their metadata, running this will access the mirrors and might take some time depending on your connection and CPU speed.
yum makecache