The tutorial you listed is quite old. It actually has a line in it stating:
Note: currently Chromium only works on the older Debian Squeeze image. It is not supported uptream for Debian Wheezy. Midori is still recommended for Wheezy.
I'm assuming that you're using either Jesse or Wheezy, which means that Chromium isn't officially supported through the package repo. You should either use Midori as they suggest, or try a tutorial written for a more updated version of the OS. Like this one.
In summary, the lines:
sudo dpkg -i chromium-codecs-ffmpeg-extra_45.0.2454.85-0ubuntu0.15.04.1.1181_armhf.deb
sudo dpkg -i chromium-browser-l10n_45.0.2454.85-0ubuntu0.15.04.1.1181_all.deb chromium-browser_45.0.2454.85-0ubuntu0.15.04.1.1181_armhf.deb
are what you're after.
As our wonderful moderator, Steve, has pointed out, there is a security concern when downloading files from a random person's dropbox.
The poster states that he's simply using his dropbox as a mirror since the files have a tendency to disappear. On the forum linked (which is the official foundation forum), it has been well received, and no concerns have been raised.
Install at your own discretion.