The Printer robot with a pen inserted
This photo shows the pen inserted into the Printer robot's carriage. The orange L-shaped piece at the top is used to hold the pen firmly to the paper; it can be moved and adjusted in order to fit virtually any pen length. Also in the back (not shown) are two bushed friction pegs (I'm using the author’s naming scheme) that work with the rubber band-driven clamping mechanism to hold the pen and keep it from moving side-to-side inside the carriage.
For more about the book, read my Geekend post Geek Gifts 2011: The Unofficial LEGO Mindstorms NXT 2.0 Inventor's Guide.