Dumb question to most of you but I can't find the answer. I am going to use the barrel connector for my project and I'm not sure what points I connect positive and negative. Below is the link. Any help or direction is much appreciated.
Why don't you plug your power supply in and then use your voltmeter to see which output pin is which?
Or, with no power supplied use the ohms setting to test for continuity between the centre pin on the input and the output pins.
I don't have one handy, I've recently moved. I was hoping to look at the datasheet and find the answer but I couldn't figure it out.
Yeah I didn't see a diagram in the datasheet showing the circuit either, and assumed you had checked, hence my suggestion.
If u have an led, resistor and a battery you can use that to make a circuit like with an ohmeter.
The majority of the time the center is positive.
Arduinos are center +
.
If u have an led, resistor and a battery you can use that to make a circuit like with an ohmeter.
And the flat side of the led is the cathode.
LarryD:
The majority of the time the center is positive.
Arduinos are center +
.
I don't think that's what he's asking: I think he wants to know which of the output pins underneath the jack connect to to the pin and the ring. That's the way I read it, anyway....
I would have to buy an LED, I didn't order one for my project, ok, center is positive, what about the other two pins on the other side of the connector, which one is negative? Sorry, this is my first time playing with electronics.
Right.
If they look at the back of the connector, usually you can tell where the center goes.
The others are the negative. One connects to the plugs side the third connects to the 2nd but disconnects when a plug is in the Jack.
As Jim mentioned, you really have to ohm things out to be sure.
.
It's not so much that the centre is positive, although it usually is. What you need to know is which of the connectors underneath where you attach the wires, is connected to the centre pin, whether it's positive OR negative..
Only way you can check that in the absence of info in the datasheet, is to do what I suggested.
PS.... unpack your meter before you go much further 
The one I use: http://www.mouser.com/ds/2/253/KC-301287-195995.pdf
Shows which is which. Center pin is A, outside wiper is C.
If you don't have a multimeter, take up knitting.
Seriously! 
OK, that is the centre pin connection, which is generally positive, except on old Panasonic junk. If I have a power pack with centre negative, I cut the plug off and if I come across a device with centre negative, I open it up and rewire it.
The three pins on the barrel jacks are center (which as Paul_B noted, ought to be used as positive), outside (ground, if center is positive), and insert detect (connected to outside of the connector when a connector is plugged in - or when a connector isn't plugged in - one of the two. I'm not sure whether there's a standard on this). You should really have a multimeter - they can be had cheaply, and it is an essential tool for working with electronics - measuring voltages and checking resistance (usually just to check continuity) are very common things to need to do. You'll notice that almost every set of instructions or electronics tutorials assumes you have one.
Whiterk:
Sorry, this is my first time playing with electronics.
We can all see that, since you don't have a multimeter at the ready, nor an LED lying around
If you don't have a multimeter, take up knitting.
Actually I was going to say "So how hard is it to get a LED ? (Thank God for Radioshack. They still have an open store in Los Gatos, CA. What ? No Frys ? Where do you live (on the Serengeti) ?
Paul__B:
If you don't have a multimeter, take up knitting.Seriously!
...but don't forget to buy needles.
The point here is that this is the General Electronics topic , so posting here if you don't have a meter is not necessarily unreasonable, given that there may some reason like for example , you left it at work, or you loaned it to the neighbor. However, on the other hand, if you find yourself at a point in your life where you decide to take up electronics as a hobby, or build electronic circuits, the FIRST thing you should do ,is purchase a meter. To make an analogy, what you are doing is like someone posting on an automotive forum , asking how to tune up their car , and not having even the most basic automotive tools. The DMM IS the MOST important tool in electronics. If you can afford an oscilloscope, even better, but it can easily be said that an oscilloscope is beyond the reach of a great many people in the world. Not everyone lives close to a surplus store where they can get a working 20 Mhz used scope for under $100 like I did. It is quite another matter to say that a DMM is beyond the reach of most people.
They are so cheap these days, there is really no excuse for not owning one if you undertaking electronics projects.
But if you're in Los Gatos, it makes more sense to go to Halted rather than the Rat Shack!! 
Harbor freight has a cheap multimeter for like 6 bucks. It will serve it's purpose in this case. Hahaha. I said Harbor Freight. I would rather take up knitting in your case until you have time to unpack. Some things are worth waiting for. My barrel jack looks slightly different than the one pictured above. MINE: The pin in the back is positive, the side post is negative and the third one underneath is just to help stabilize it on a breadboard. NO! I have a multimeter and checked it out, it's negative as well. Now, I learned something primitive.
Halted is where I got the scope, and it's too far away for small items RsdioShack, on the other hand is 3 blocks away at the Nob Hill Shopping center That's where I get solder and proto boards , wire and other small items.
FYI, Halted moved.