You can find the app in the 1Mobile app store. It might be better to download that app store's client app before trying to get the ADW Launcher app, though you can use the second link at the bottom of the post to download the apk install file instead.
Once you've downloaded it, you might want to look at the optional alternate themes. a number of developers have released a theme which works with ADW Launcher to change the look of the home screen. Here are a few options:
On a related note, I have also updated my post on 5 free apps for the KFHD so it includes this app.
But as great as ADW launcher is, it's not a perfect solution. The home button on the bottom taskbar will still take you to the original Kindle Fire Home Screen, so you might want to add the icon for ADW to the favorites menu. That will put ADW only 2 clicks away.
Using ADW Launcher is also going to reveal several apps that Amazon keeps behind the scenes. And this launcher doesn't quite work correctly with some KFHD features, including the Kindle app and ebookstore. For those you will need to go back to the original home screen before opening.
All in all, this is a big improvement. I have never liked the design of the KFHD's carousel style home screen and I have long wanted to replace it with an alternative that looked more like a traditional Android home screen. And now I can.