Did you try witching the state (off/on) in the preferences?
Perhaps the preferences file is corrupted.
You can try resetting it as follows:
1) Close Usenapp
2) Open Finder
3) In Finder go to Applications/Utilities and open Terminal.app
4) Type the following command in Terminal.app and press enter:
defaults delete com.xoroxsoft.usenapp [enter]
5) Close Terminal.app. You no longer need it.
6) Open Usenapp.