The installation media is now a hybrid ISO image, working in two ways:
Using USB sticks is faster and more environmental friendly and therefore the recommended way to install Proxmox VE.
In order to boot the installation media you need to copy the ISO image to your USB media. You need at least a 512 mb USB stick.
NOTE: Using UNetbootin does not work.
You can simply use dd on unix like systems. First download the iso image, the plug in the USB stick (you need to find out what device name gets assigned to the usb stick).
# dd if=pve-cd.iso of=/dev/XYZ bs=1M
Be sure to replace /dev/XYZ with the correct device name (be careful, and do not overwrite your hard disk!)
Connect your USB media to your server and make sure that the server boots from USB - and follow the installation wizard.