Select a device and a keyboard key on the left, then click a USB key or special key to assign it.
Keep in mind that while keys are defined in the USB specification, they may not be recognized by your operating system. For example, Keypad = is not recognized in Windows but does work in Mac OS.
Locking keys and other odd keys (Mac and ADB caps lock, ADB power key, PS/2 pause) may not behave as expected.
Download the layout utility here to upload the layout file to your device. You'll also need the latest firmware.