The Download Now link directs you to the Mac App Store, where you can continue the download process. You must have an active iTunes account to download the application. This download may not be available in some countries.
From Dave Sperl:
UnderTheWX reads weather data off the popular AcuRite USB Professional Weather Centers and uploads it to Weather Underground, Weathercloud. net, PWSWeather and WOW. The app is a Status Bar Menu which displays Temperature, Humidity, Wind speed and direction, Rainfall and Barometer. UnderTheWX supports the AcuRite 01025, 01036, 02032, 02064 and 06037M USB Personal Weather Stations. Uploading data to Weather Underground is also supported using the latest "Rapid Fire" update feature.