your security perferences allow installation of only apps from the mc app store and identified deleiopers?

1. Launch System Preferences from the Apple menu.
2. Choose “Security & Privacy” and then click the “General” tab, followed by clicking the lock icon in the corner to unlock the settings.
3. Look for “Allow applications downloaded from:” and choose “Anywhere”.
4. Accept the security warning and allow.

Note that you should be careful and aware that now the job of protecting the system from malicious apps belongs to you instead of the OS.