When I recently installed Xubuntu 13.04 and added PHPUnit, apt-get installed version 3.6. This is not unusual; the repository is usually running an older version. I happened across this article on upgrading to a newer version and am posting it here (link to original article is at the bottom). All commands should be run via command line, of course:

sudo apt-get install phpunit
sudo pear channel-discover pear.phpunit.de
sudo pear channel-discover components.ez.no
sudo pear channel-discover pear.symfony-project.com
sudo pear upgrade
sudo pear upgrade phpunit/PHPUnit

This upgraded my installation from 3.6.x to 3.7.24 (as of this writing)

Source: http://konradpodgorski.com/blog/213/how-to-install-phpunit-on-ubuntu-11-10-and-get-it-to-work/