The STM32F3-DISCO does not provide a VCP via the ST-LINK.
Only newer DISCO boards supporting mbed, and the NUCLEO boards, have the VCP functionality.
My purpose of using USB USER port is USART communication as Serial communication.
My STM32F3-Disco will calculate many floating point.
And i want to see the result on my notebook.
How can i see the result on my Notebook?
You'd have to code a USB VCP for the F3 itself. Look at the USB FW library, or Cube to get that done.
Consider the NUCLEO F3 board.
if you are using STM32CubeMX, you need to enable USB and select "Communication Device Class" under MiddleWares --> USB_DEVUCE --> Class For FS IP.
Also see this Stack Overflow Issue.
If using CubeMX and using the CDC stack, Windows should show you a Virtual Com Port (not ST-LInk) in your device manager.
Edit: Ups, just saw that this was an old entry. But maybe my answer helps someone else ...