Recently I bought a MacBook Pro 13″ with Retina Display and the first thing I wanted to do was connect my Casio CTK-800 keyboard to it and use it with Garageband just like I saw in this epic video where these guys jammed out on such a retro set up! source – http://progamerreview.com/
The driver for Mac OS can be found here and even though it specifically states versions 10.7 and 10.8, it will work on 10.9 and presumably later versions of Mac OS when they’re released.
If the link is broken, I have also uploaded the files for downloaded on this sites Amazon S3 bucket here.
All you need to do is download the package and then install it. Then plug in your CTK-800, open up Garageband and start hitting keys and it should just work.