FiiO

Is the M3K showing the charging icon when connecting to the computer?

For storage mode:

Please check whether you have inserted the SD card first. 

If yes, please go to system settings of M3K first. And check whether you have switched the usb mode to storage in the M3K?

If the issue still remains, please try another usb cable and port,SD cards as well.

For DAC mode:

Is your computer the Windows system? If yes, you would need to install the DAC driver first:

http://fiio-file.fiio.net/FiiO_USB_DAC_Driver-V4.47.0.zip 

Is the player showing the charging icon when connecting to the computer?

Which usb mode are you choosing in the player? 

For storage mode:

You would need to install the MTP driver first.Please read this post for help:

For DAC mode:

Is your computer the Windows system? If yes, you would need to install the DAC driver first:

For more information, please read this post for help:

If the issue still remains, please try another usb cable and port as well.