In Part 5, "Configure the network", the author of this very useful guide forgets to mention something:

Then restart your network:

/etc/init.d/networking restart

What he doesn't mention is:

You're not supposed to type that '/etc/init.d/networking restart' in vi          The '/' that takes you to the bottom of the screen is the start of the search function in vi.

First save your work (press <esc>, press the colon and type wq followed by <enter>) and at the command line type the command.

It's true, because I was stuck until I emailed the Ubuntu Forum, and Wim Sturkenboom kindly explained me what I was doing wrong.

Hopefully you'll find this useful, too.  (Thank you, Wim!)