Hello Randy,
I'm confused about your question , now let's confirm some points.
1) Is there a different way to debug using mbed ?
-> No, the method is the same no matter debug probe is MBED, JLINK or P&E on MCUXpresso IDE.
2) hold down the reset button, and reinsert the USB cable
-> If your board is FRDM-64, this action is force debugger on board run into bootloader mode, then we can update
debugger firmware, for example J-link, P&E, MBED. About detail steps please refer to : OpenSDA Serial and Debug Adapter | NXP
3)Now, I recommend you update it to J-link debug probe refer to above link, in my experience, sometimes it can't work
well MBED on FRDM-k64 . Remind again, if your PC is WIN10, pay attention the notes:

4) If still can't debug , please take screenshot show your steps.
Have a great day,
TIC
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