When attempting to boot from the VMWare vSphere Hypervisor installation CD, it stopped at the syslinux boot screen complaining about “No DEFAULT or UI configuration directive found” and shows only “boot:” prompt.
The solution is simple, type the following in the prompt:
mboot.c32 -c boot.cfg
which is found in isolinux.cfg.
Seems like the bootloader is expecting syslinux.bin and syslinux.cfg instead of the isolinux pair in the CD. Alternatively, you may rename the files and create another CD/USB.