You want it to be master. If you choose one of the USB APC upses then you will get the usb_hid driver. But there are various parameters that differ depending which UPS you choose. The Freenas instructions say you have to enter the right USB device from the console message you quote. On Linux you don't need to say which USB port, but maybe you do on FreeBSD. You will have given your UPS a name, and if, at the command line, you type upsc yourupsname <return> you will get a screenful of data if the driver is working. If none of the APC USB drivers work the next place to look is the NUT homepage. I can't remember if they have a forum, but they do accept queries.