I found the guide below when trying to boot a VMWare Workstation virtual machine from a USB stick with a bootable image of XenServer that I created using the great utility UNetbootin.
VMWare does not have an option in the BIOS to boot from a USB device so in order to accomplish that I followed the instructions below by Vladan Seget which explains how the use the Plop boot manager.
01. Download boot manager from PLoP
02. Use the Iso in the boot manager as a boot device to boot your VM
03. Add an USB adapter to your VM (if not already done).
04. Insert your USB stick with ESXi 4 installed in it.
05. When your VM boots from that ISO, just select USB from the menu.