Today I needed to spin up a VM with CentOS 5.5. My preferred distro these days is Ubuntu, and I had to learn the CentOS dance to get an unsupported version installed, so it took me a little longer than I would have liked to get it going. Here's the secret:
When asked for the name and directory of the install image, enter vault.centos.org for the name and 5.5/os/x86_64 for the directory. If that doesn't work for you, go back a step and disable IPv6. After that, you should be good to go.