First, get the VM from PuppetLabs.
Unpack the .tar.bz2 file so you see the learn_puppet_centos.vmwarevm directory.
Start VirtualBox, and create a new VM. Set it for Redhat Linux, not 64-bit. 512mb RAM is fine.
Add the cent55_386_ks.vmdk or a dummy image as the hard drive.
Save the VM, but don’t start it yet.
Edit the settings for the VM, and remove the SATA controller, and add the disk image to the IDE Controller. The VM only supports the PIIX controller, so the AHCI SATA controller that VirtualBox 4 uses by default won’t work.
Boot! Enjoy! The login is root and the password is puppet.