I think the problem is that Wunderground only released a Windows version of the WeatherLink plugin -- I've never seen a Mac-specific version.
A couple of releases of WeatherLink ago, Davis decided to remove WeatherUnderground upload features from it's product in favor of offering developers an API to build 'modules' that could plug-in the basic WeatherLink.
WeatherUnderground developed the module for the PC/Windows platform, and added Rapid-Fire support for it earlier this year.
I'd suggest you post the question at WeatherUnderground in the blogs area for the developers and see if they have plans to release a Mac-specific plugin for WeatherLink.. Davis isn't doing that kind of development as far as I can tell.
Hope this helps!
Best regards,
Ken