Is there a "Find My Location" button or such on the settings? I didn't understand well from the FAQ, thanks!
All you have to do is type in your Zip Code. If you don't have one, then type in your city name and click "Search" (or press Tab and then Enter instead of clicking Search).
Or, go to Google Maps, and then right-click your home and choose "What's here?". Then, copy the coordinates (the latitude and longitude) and paste them into f.lux.