It is simpler than it seems, just get to a terminal, become root, and install the RPM with this command:
rpm -Uvh http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-3.noarch.rpm
After the install, you should have a working EPEL repo, I use it all the time, this is also a requirement for RPMFusion on CentOS, too. Anyway, one last note, you might want to install foo after, but it works for me without it:
This command is not really needed for some reason, but if you run into problems, you will at least know hat you can try. Hope this helps, Ben.