The link you've posted is broken - is it meant to be
http://www.ubuntugeek.com/howto-setu...in-ubuntu.html?
I don't know much about TFTP, but it sounds like there's some kind of problem connecting to the other computer. Can you ping it?
(i.e
replacing the IP address with the IP address of the computer you're trying to TFTP to)
Hope this helps,