Hold the back button til it reappears. Hold it again to make it disappear. To disable all together, go to settings, my device, display, and uncheck multi window.
The advice you got is the fastest answer, but you can also pull down your tool bar, click the icon that looks like 3 squares and something else, and tap "multi-window" to enable it. You may find that you accidentally turn it off from time to time by toggling with the back button--that may be how you lost it in the first place.