I found a great fix to be able to create use the ESXi installer .iso and copy it to a bootable USB. Neither my MBA nor my physical host has an optical drive. I found the solution on the Mac Repairs Lond blog but I wanted to summarize for posterity:
Format the USB drive using DiskUtility with FAT and a MBR.
Open the terminal and find the mount point of the USB drive
Then unmount the disk and use fdisk to mark the partition as bootable
diskutil unmountdisk /dev/disk#
fdisk -e /dev/disk#
This enters fdisk interactive mode so enter the following commands to mark the partition and write the changes.
<code>f 1 write quit</code>
At this point I mounted the USB drive and copied all the contents from the .iso to the USB drive.
One last step, find the file
ISOLINUX.CFG and rename it to
SYSLINUX.cfg. Then edit the file and find the line that starts
APPEND -c boot.cfg and append
-p 1 to the end. Save and Exit. You’re ready to stick the USB drive in and boot the ESXi installer from USB.