Answer
We do not support the USB interface on the Sterling-EWB. The VDD_USB power supply pin ,as labeled, only provides bias to pins PA11 and PA12. That is why it specified as 3.0 – 3.6V in the datasheet.
The USB interface on the DVK (J24) is only to the FTDI USB-UART translator, the communication to the EWB is on UART1 from the FTDI part.