CentOS - Aug 2008 - extlinux on centos 5.2

I am trying to use extlinux to boot a usb device.
I ran the following on the device:

mkdir -p /boot/extlinux
extlinux /boot/extlinux
cat /usr/lib/syslinux/mbr.bin > /dev/sde
cp /boot/vmlinuz.latest.kernel /boot/extlinux/linux

I then attempt to boot it and it starts going however
I says kernel panic no root device.

What have I missed. I followed the instructions in 
/usr/share/doc/syslinux/extlinux.doc

Thanks,

Jerry