Mac: When I try to open the SubMachine .pkg it says that it’s from an unknown developer?

In the Finder on your Mac, locate the app you want to open.

Control-click (or right-click) the app icon, then choose Open from the shortcut menu.

Click “Open.” The app is saved as an exception to your security settings, and you can open it in the future by double-clicking it just as you can any registered app.