You need to read over this project carefully - the author does a good job of warning you about the risks to your DC motor if you don't take care to provide the right amount of power to the motor. That said, I ordered a tiny motor that the Arduino is capable of providing power to (without an external power jack plug specified in the hardware listing) as long as the 10K potentiometer is thrown into the mix.
If you're not certain what you're doing, then wire up the project exactly as described in the book - he shows two AA batteries providing the power but if you have a variable voltage wall adapter, that could likely be used if you set the voltage to a level that matches your DC motor specs.
So... as you can see from my photos (and video), I've skipped the power jack and I have my DC motor getting power directly from the V+ line on my breadboard that, in turn, is getting power from the Arduino when plugged into the USB port. Maybe over time this would be a bad thing for the little motor, but for short bursts like in the video, and with the 10K cranked up a bit, it's not a problem. (And at $1.95 each, if the motor dies I won't lose too much sleep.)
I did use the specified 1N4001 diode in my circuit as well as the TIP120... I also removed the MakerShield to give myself a bit more room on the larger breadboard to spread out and wire it all up. I could easily have wired all this up on the mini-breadboard on the MakerShield now that I look at it, but I liked having the room to spread out.
In the video, when I apply power, there's a slight hum coming from the motor... only when I crank the 10K down a substantial amount does the motor start to spin. I put a piece of tape on the motor shaft so you can see the spinning action... it happens fast in the video before the tape is spun off... so watch carefully. Also not sure if you can hear the hum of the motor, but it's there, too.