The USB logger has two modes, virtual serial port, and USBXpress. Weatherlink software (at least on the PC) can (I think defaults to) setting the logger into USBXpress mode, but WD can't use that, so (at least on the PC) you have to be careful to set/reset the logger to virtual serial port and be sure that the Weatherlink software is also set to virtual serial port. The logger hardware is the same for PC or mac, but like Budgie, I don't know if USBXpress is only a PC thing.
I moved the thread here to the mac section so hopefully you'll get some more mac-literate users to help you.