It's called chromeTouch, which you can download here. After it's done installing, it's pretty straightforward and easy to set up. You see a new icon in the address bar (which you can remove):
Now, click the Settings button, and go to Tools -> Extensions (I'm on the beta, and it could be different on the stable version, but you need to get to the page with all your extensions and the settings for them).
Here, click the Options link under chromeTouch. You can set all the various things. Remember, let the mouse button setting be 'Left', because it won't work when you scroll using your finger (that's a stupid setting, right?). I also recommend keeping the Touch Mode to Auto, which means that the scrolling won't work when the pointer is on text (for selection). You can change it easily by clicking the icon in the address bar too.
Rest of the settings are fine by default. Enable the mouse gestures thing if you want. You will be able to go back or forward by flicking left or right.