This comes up because I'm not a part of Apple's developer program. As long as you only get StimuWrite from official sources (this page currently is the only place), you'll be safe.
Try this:
- Drag the app into your applications folder.
- Open the Finder and navigate to the application.
- Right click or control click the StimuWrite icon and select open.
- Click the Open button that comes up and it should open and not give you a warning anymore.
You can find more info on the apple website here. I hope that helps!