Powering the board with a DC Barrel Jack adapter

Just getting started, and no background in electronics.

I am following this tutorial on Lifehacker:

On the instructions, it suggests using a DC barrel adapter to power the board,

However, the adapter does not fit in the space suggested in the article:
http://cache.gawkerassets.com/assets/images/17/2012/01/9b19b1171c33b361d8cbf907bb36ee1a.jpg

See it there on the end? It’s designed to fit in the middle of the board, not the end, so all 3 prongs fit. How do I make this work?

Any advice is appreciated -

I'm not sure what they are trying to show.... In the picture it looks like there is some sort of adapter board.

I dunno.... You may have to solder a couple of wires to the barrel connector. There are only 2 wires coming from the power supply (+ and -, or +voltage & ground). You only need to use 2 of the 3 terminals, but you need a way to figure-out which terminals to use and which way to connect them.

If you don't have a multimeter, you can make a power & polarity tester from an LED and series resistor. Test your "tester" on the Arduino to find out which are the +side and -side of your resistor/LED. It doesn't matter if the resistor is on the +side or the -side, as long as they are in series and as long as you figure-out which way makes the LED light-up.