Firstly, try to open the PixMiller App. You’ll see a notification that it can’t be opened. Then, follow the instructions:
Open System Preferences.
Click on Security & Privacy, then go to the General tab.
You’ll see the message “[App] was blocked from opening because it is not from an identified developer.” Clicking the button next to it marked “Open Anyway.”
You’ll be asked again if you’re sure you’d like to run the app. Click the Open button to run it.