When you install WeatherCal, it doesn't end up in your Applications folder. Instead, it's a preferences pane that appears in System Preferences (below). You can add as many cities as you'd like to the list, but adding too many cities might make your calendar unreadable.
Each day's forecast is visible in iCal, with a small icon representing the expected weather. In the screenshot at the top of this post, you can see that just when I'll be volunteering for the FIRST Robotics Colorado Regional, snow is expected in my area. Opening the forecast in iCal provides a link to a detail page on the Weather Underground website:
I like the premise of WeatherCal in that I can now remove other weather apps from my iPhone and just look at my calendar. It's also nice to be able to know what the weather is going to be like on days when there's something important going on. For $10, WeatherCal is a very useful little tool. A 30-day full trial is available here