Hello Andrew:
I would like to help. Please answer some questions:
1) Do you see something under Ports (COM & LPT) in Device Manager?
2) The FRDM-KL43Z has 2 USB ports. Which one are you using?

3) Do you see the green LED (D8) ON when connecting the board?
4) Cannot you enter BOOTLOADER mode? This is done by holding reset button pressed and connecting the OpenSDA USB port.
5) Have you tried with a different computer, especially one with Windows 7?
6) Does you PC have an antivirus? If so, try disabling it momentarily.
These are just generic questions, I am trying to understand better your issue, so please be specific.
Regards!
Jorge Gonzalez