Yesterday I downloaded and installed iStat Pro from this website. Once installed, a window popped up and said it is located in the Widgets folder. After searching in my Widgets folder, I was unable to find it. I also searched through Spotlight and through Finder.
It's located in your ~Users/Library/Widgets folder. While in dashboard, you need to click the '+' icon in the lower left of the screen and select the widget to activate it.