Everything I see on the web site for that free book (i.e. in their help section about mobipocket format) says that you need to plug your PID into your mobipocket software, which is what does the download. I imagine that it encodes with that PID then.
Since I'm on a Mac, I can't run the computer end software, so I can't get books that way. Anybody else down load this already, and can give us more insight?
ADDED: in the original thread
TheLongShot is having problems with the mobipocket version too. You might ask over there. If there is NO DRM, you don't need your Kindle PID. Just read it!