I can get the Rapid IoT Prototyping Kit into download mode using either of the two methods shown, but it is not enumerated as a mass storage device either on my Windows 10 laptop, or on the Ubuntu 18.04 desktop. Neither computer gives any sort of indication that a USB device has been added or removed regardless of whether the Rapid IoT is in dowload mode or normal mode. It does not show up in Windows Device Manager, or Linux lsusb.
Please find a video exaplaning the sequence of action to enter mass storage mode:
https://community.nxp.com/docs/DOC-342335
Make sure to keep hloding for 2 sec SW3 button after releasing reset.
If this still doesn't work you can try to recover default binaries of rapid-iot:
Recovering k64f & kw41z applications binaries
https://community.nxp.com/docs/DOC-342112
Regards,