Launchpad, the iOS-style apps launcher feature that appeared in OS X Lion, showed up without a keyboard shortcut enabled for it. Later Macs, of course, have the F4 key assigned as a Launchpad, well, launcher. You can use the dock icon, of course, and you can double click the Launchpad icon in the Applications folder, but if you have an older Mac keyboard, here’s how to enable the F4 key (or any other shortcut you want) to access Launchpad easily, with the touch of a keyboard button.
Open System Preferences from the Apple menu or the Dock and then click on the Keyboard preference icon to access the Keyboard preference pane. Click on the Launchpad & Dock label in the left-hand column, and you’ll see two options appear on the right. You can allow the Dock to be hidden here as well, but for the Launchpad, you’ll need to click on the checkbox next to Show Launchpad. The shortcut field will activate to the right, letting you hit any key or combination of keys on the keyboard to activate Launchpad.
Also, for example, if your F4 key is set to activate a different feature like it is on my older Apple Bluetooth keyboard (the F4 key launches Dashboard), you can set Launchpad to activate with a different F key, like the F5 key. Simply hit that key on the keyboard when you’ve double clicked into the currently set shortcut for Launchpad in the system preferences pane.
Via: Macworld Hints