@pierrot10 which model of the expansion board arte you using. Version 2 (one push button) or version 3 (two push buttons). Version 2 is more simple. If you cannot see the serial port, it is a problem of the physical connection (cable, jumpers on the board) or the OS X driver. Version 3 adds to the list of trouble sources the firmware in the PIC controller on the board. I assume that you also do not get a REPL prompt.

@combaindeft B) is clear, since the 3.3 regulator is on the FiPy device. Without that, no power at the 3.3V pin. Due to A) with the rock solid supply through a battery it is hard to accept, that it's a firmware issue, at least of the FiPy. Still, the PyTrack board can be the culprit. Did you try to run the FiPy without the PyTrack board, like on a breadboard with a separate USB/serial interface? Shorting 3.3 and GND should be avoided. P.S.: It's a pity that Pycom does not publish schematics.

@jonaswg I never did that with a newer xxPy device. But I used Wipy 1 in that mode with a Debian Linux computer. If you terminal emulator has flow control, switch that off. The other settings are 115200 baud, 8 bit, no parity. If you push reset at the FiPy, you shoudl at least see some boot messages.

For the record, there is now a socket.recvfrom(bufsize) method that will return a (data, port) tuple.