From the link you posted to: brool.com/index.php/installing-centos-on-an-external-usb-drive
"2. When you reboot, reboot to the CentOS CD AGAIN and type “linux rescue”."
When you boot your computer to an install/Live CD of some Linux flavors (Ubuntu is like this) you are presented with a menu that asks you what you want to do (install for different locales, test memory, edit boot options, etc...). This sounds like the place where you need to be.
So I'm thinking that you need to boot into your CentOS install disc. From there, you should be able to enter your "linux expert" command at the installation boot prompt.
I found a screenshot for CentOS5...not sure what it looks like for 6.