Dear Fusion2u and WeatherCat fans,
First welcome to the WeatherCat forum!!
Unfortunately, folks are having a lot of trouble trying to use serial to USB converters as Apple keeps changing OS-X. There have been a number of threads on this forum recently where folks have run into similar problems. Unfortunately I went with the USB data logger so I just haven't kept up with these threads. However if you do some quick searches you should be able to find out what have been the recent successes.
If that doesn't work, you might consider getting another serial to USB dongle that is known to be supported. That isn't too expensive and if you pick a model which has good Mac support that hopefully would be the end of driver voodoo.
Hate to suggest the alternative and it isn't foolproof either, but you might also consider upgrading to the Davis USB data logger. It is expensive but is more reliable and so far Silicon Labs has been doing a good job of keeping up with Apple's OS upgrades.
Sorry for your predicament. It isn't fun when Apple shuffles your life with each OS upgrade.
Cheers, Edouard