Applies to Launchkey [MK3]

This can either be done on a Mac system or an iOS system. See this article on how to use a Launchkey with an iOS Device. 

Open GarageBand, create an empty project, and create an instrument track:

The Launchkey would then be automatically detected to work with the track. This will mean you can use the keys to play notes, use the Arp, Fixed/Scale/User Chord Modes, and pads for note messages in Custom and Drum modes. 

Additional control can be achieved by installing the script made for Logic, though the script itself is built for use with Logic rather than Garageband. The script can be downloaded here.