Hi xuzhong nie,
By default, the JLink debugger interface which also include the CDC feature is programmed in FRDMK64F, when the board is connected to your PC via USB cable (please use J26, SDAUSB port to connect your PC), you will see "Jlink CDC uart Port (COMx)" in "Device Manager".
Then, you can use this USB virtual port to debug.
Hope it helps!