If you ever face "kernel panic" issue when trying to boot Centos 6 from your USB stick, it is due to the EFI bootloader doesn't point to root to your USB stick.
To fix this, simply go under the EFI folder in your USB stick, then find those files end with *.conf, use a notepad to change the "root=" to your USB device. In my case, it is some things like "live:UUID=xxxx".
Another note to take care is instead of install using UNetBootIn or ISO2USB, we can actually use Fedora Live USB Creator. The good thing about use Fedora's is that you can find your USB device's UUID under syslinux\syslinux.cfg.
Wish you will have a smooth boot!