with Mac OSX 10.11.4, which didn't recognize the BU-353s4 receiver first, the solution was a restart of the system and a couple of minutes of patience, until the signals of the receiver showed up.
With Win 10 on Parallels 11 it also was a restart and a replugging of the receiver until a window popped up, which allows to decide to which of the computers
the device shall be connected. After connecting to Windows the port was visible in the options window of openCPN and I could click apply and it worked. Huuuh.