1. First, install and try to open the Numbas app at least once. If you are asked to move the app to the bin, choose "Cancel".
2. Click on the Apple icon in the top left of your screen and select the "System Preferences" option.
3. Select "Security & Privacy" from the "System Preferences" window.
4. Select the "General" tab, and click the lock in the lower-left corner to allow changes.
5. Enter your computer username and password, then select "Unlock".
6. In the "Allow apps downloaded from:" section, select "App Store and identified developers".
7. The Numbas app may also appear below this section next to a button labelled "Open Anyway". Click the button if so.
8. Close the window. You can now install third-party applications that you trust and should be able to launch the Numbas app.
9. The first time you open the app after this procedure must be done via right-click. Open Applications in a Finder window. Find the Numbas app, then right click it (click with two fingers on a Macbook touchpad). Click Open. When the warning message appears, an extra button labelled Open should be available. Click it to open the Numbas app.