The USB port is connected on PB15 and PB14 (OTG HS).

To enable the Serial on this port, you have to configure the compiler options in the Arduino IDE :

In "Tools" choose :

• U(S)ART support : Disabled
  
• USB support : CDC generic serial
  
• USB speed : High speed in full speed mode
  

Now the board should be reconized as an USB device