Setup takes about 20 seconds, one time only. After that, the app opens normally every time.
Double-click TheAccountant-Setup-64bit.exe.
Click the small link "More info" in the blue box. A "Run anyway" button appears at the bottom — click it.
Click Next / Install through the wizard, then Finish. TheAccountant opens automatically.
From now on, open TheAccountant from your Desktop or Start Menu like any normal app. No more warnings.