Besides the trial period of 10 days which may have expired, this symptom happens sometimes if the preferences file where SwitchResX saves its settings has been corrupted in any way.
You can solve this problem, most of the times, by removing the preferences file. Just put the file, named fr.madrau.switchresx.daemon.plist, in the Trash. The file lies in the Library/Preferences directory.
If you're on Lion, you may not see the Library directory. In this case, press the option key (also named Alt), and select "Go" in the Finder menus. You'll have the option to select the Library directory there.