After installing Mountain Lion on my mid-2011 iMac i realized that the launchpad key (F4) that my MacBook Air has, didn’t exist on my Apple wireless keyboard. No worries, you can assign a keyboard shortcut to open it.
Click on the Keyboard in System Preferences, then choose Keyboard Shortcuts tab along the top.
Click on Launchpad & Dock on the left side of menu. Click Show Launchpad, press Enter or Return, and enter your shortcut. Once you’re satisfied, you can click OK and close all windows.
All done. I set mine up to use F4, same as it is on my MacBook Air. Force of habit since I use it often.