i am having the same problem on my Z600, tried installing it today.
Did you find a solution for this?
After contacting VMWare support, the answer I got was, HP Z600 is not on their harware compatibility list.
just to let you know, I installed citrix Xenserver without any prroblems on my Z600...
We've successfully installed ESXi 4.1 Installable Update 1 (CD ISO) on an HP Z600. We installed to a USB flash drive. The factory-installed optical drive was not supported by the ESXi installer. An external USB optical drive was used instead. See the attached PDF for detailed instructions.