How to install stacks
Accidentally I deleted stacks , how to get it back
MacBook Pro (13-inch, Mid 2012), OS X Mountain Lion (10.8.2)
Drag it from the Finder onto the Dock. You need to drop it on the right side of the divider. When you hover over a gap between two items, it will expand to make room. Drop it there.
Then, right-click on it and choose Stack from the Display as options.