Insert the MIDI cable into the "MIDI OUT" jack on your keyboard. Check your keyboard's user guide for the exact location of the jack. The MIDI cable should have five prongs that match up with holes inside the output jack. Guide these prongs into their matching holes. Wiggle the cable after inserting it to make sure it sits securely.
Insert the other end of the cable into the "MIDI IN" jack on the MIDI interface device. Check the interface's user manual for the exact location of the MIDI input jack. The jack will have 5 holes to match up with the 5 prongs on the MIDI cable.
Connect the MIDI interface to the Mac Book Pro with a MIDI to USB cable. Insert the MIDI end of the cable into the "MIDI Out" jack on the interface. Insert the other end into an open USB slot on your laptop.
Turn on the MIDI keyboard and Mac Book Pro. Plug the MIDI keyboard's power cord into an electrical outlet. Press and hold the circular button to the right of the Mac Book Pro's keyboard. Wait for the laptop to power up.
Open the Garageband music application and wait for the program to recognize the keyboard. Open the Garageband application on your computer and let it start up completely. A window will appear stating you have a new MIDI device available for use.