Welcome to the Apple Support Communities
What happens when you try to install the widget? If you get a message that the app comes from an unidentified developer, Gatekeeper is preventing you to install the widget > http://support.apple.com/kb/HT5290
To install the widget, there are two ways:
1. Right-click the installer and choose "Open".
2. Open System Preferences > Security & Privacy > General, and choose "Anywhere" under "Allow applications downloaded from"