When you launch the app, it asks for your root password. This isn't stored and is required to write to the SD card, so it's safe to provide.
The app detects any SD cards connected to your Mac, simple select it in the 'Pi-Crust' section, then select your .img file in the 'Pi-Ingredients' section. To write the image to the SD card, click Restore Backup and wait for it to complete.
Once the image is written, you're notified that you can eject the SD card.
That's all there is to it. You can also create a backup of your existing SD card using this app.