On many Linux distros xinetd isn't installed by default. You'd have to use one of the debian install utilities to install it. (e.g. apt-get, dpkg etc...)
Also even if it is installed it's possible tftp isn't turned on by default (or not even installed) so you may have to install and/or configure tftp in xinetd to run.
There is surprisingly enough a howto for xinetd AND tftp at the following link:
Note that it has RHEL/Fedora AND Debian/Ubuntu - be sure to use the correct setup for Debian/Ubuntu rather than RHEL/Fedora.