If you use an old apple keyboard you don’t have a special key to start Launchpad in OS X Lion. But you can assign one in System Preferences, where you can also change existing system keyboard shortcuts.
Go to System Preferences > Keyboard > Keyboard Shortcuts. Click on Launchpad & Dock. Tick the box to the left of “Show Launchpad”. This enables the setting and moves the focus to the far right, so you can define which key you want. Simply press the key, or combination of keys you would like to use. To change a shortcut just double-click it, and press the new keys. I use F5 for mine, and have F6 set up to show the desktop. The show desktop option is located under Mission Control.
If you did not perform a clean install of Lion, but updated from Snow Leopard, you may experience that after a reboot the key does not work. The reason for this is you have old settings in your user library that causes problems. You have to remove these, which sets everything to default, and then make the changes you want.
To do this go to /Users/username/Library/Preferences/ and delete com.apple.symbolichotkeys.plist. Be sure to switch out username with your actual short username.
Since the user Library is hidden in Lion, you will have to use the Finder > Go > Go to Folder, and manually type in the path.