1. Downloaded Arduino IDE
http://arduino.cc/en/Main/Software -> "Windows Installer"

2. Download MaKey MaKey Addon

    Extract it

    Put extracted "hardware" folder into My Documents\Arduino (so it contains both the hardware and the libraries folder)

3. Plug in the MaKey MaKey

    Install drivers in My DOcuments\Arduino\hardware\MaKeyMaKey\driver

4. Install the MaKey MaKey drivers

5. Figure out the COM port for the MaKey MaKey

    Unplug the MaKey MaKey while Arduino IDE is open
    Look what's listed under Tools > Serial Port
    Plug in the MaKey MaKey
    Look what's now listed under Tools > Serial Port, and select the new options