Just drag it from the apps area to your main screen and then drag it into the bottom bar of the screen to where it used to live. All the buttons in that bar are rearrangeable and deletable/replaceable. You can put whatever you want in that bar, and even stack multiple buttons into one--for example, all of my internet apps are stacked into one button, so when I touch the button I get a box full of internet apps, instead of having to dig around in the apps area for them (and it keeps the desktop cleaner too.)