Mountain Lion Dock is a dock theme for all Mac OS X users that have a sweet tooth for interface customization and want to change their dock's default skin so that it better matches their system's overall look.
Mountain Lion Dock is a free and Mountain Lion themed, 3D dock skin for your Mac.
· Go to Macintosh HD > /System/Library/Core Services/
· Right-Click on "Dock" and select "Show Package Contents"
· Go to /Contents/Resources/
· Backup the files you are about to replace
· Move those same original files to the trash
· Move the files you downloaded to that folder
· Open Terminal and type "killall Dock" (without the quotes)
· Enjoy your nice new dock.
NOTE: Make sure you backup the files you will replace in the Dock app.