I've tried converting .sub and .idx files, but the output .srt files come out empty.
The latest version of the software is 2.0.1 and it's said it fixed some bugs within the code. If you have a different version, upgrade to the latest one using the App Store link.
App Store: https://itunes.apple.com/us/app/subtitle-converter/id568415685?mt=12
However, on extensive search I have found that the developer isn't in the business anymore which makes it impossible to obtain full support.