Since the release of OSX 10.7, your Mac is initially set up to only allow Applications found on the App Store to install on your computer as a Security Measure. Otherwise, it will consider the application to come from an Unidentified Developer and block installation.
By default, Mac OS 10.8.x , 10.9.x , 10.10.x , 10.11.x and macOS only allows users to install or run applications from 'verified sources'. Sometimes, when a known developer (such as Microsoft or PowerSchool) needs to update their application's code to cofirm their 'identity' with Apple, an error may occur. As a result, users may be unable to install or run some applications downloaded from the internet or stored on physical media, and the error below is displayed:
In the meantime, Users can follow the directions below to prevent this error message from appearing
Open the System Preferences. This can be done by either clicking on the System Preferences icon in the Dock or by going to Apple Menu > System Preferences.
Open the Security & Privacy pane by clicking Security & Privacy.
Make sure that the General section of the the Security & Privacy pane is selected. If the lock icon at lower left is 'locked', click the icon labeled Click the lock to make changes.
Enter your username and password into the prompt that appears and click Unlock.
Exit System Preferences by clicking the red button in the upper left of the window.
You should now be able to install/run applications that are tagged as being from an 'Unidentified Developer'.
Once the developer has corrected their 'identy' issue, be sure to revert changes made above, and once again allow only 'applications downloaded from Mac App Store and identified developers'.
For more information about this issue, please visit Apple's KB article on the topic: http://support.apple.com/kb/HT5290
Apple has a built-in way to protect you from opening up potentially malicious apps on your computer in Mac OS X. This setting, named Gatekeeper, will never stop you from installing apps from the Mac App Store, but it can help prevent helps from being installed that are not a part of the Mac App Store. However, if you get a security warning and still want to install the app, here is how to do it.
Open up your 'System Preferences' and head to 'Security & Privacy.'
In the 'General' tab, you'll see a section called 'Allow apps downloaded from.' In order to change these settings, you'll have to click on the lock icon below and input your admin password. Once that's out of the way, you can choose between three options:
The safest option to choose here would be the 'Mac App Store,' since Apple reviews all apps that are accepted in their store and can remove them quickly if necessary. You will know that your Mac will be as free as possible from malware if you use this option only.
The next option also trusts apps from 'identified developers,' which are developers with a unique Developer ID from Apple who have their apps signed digitally. This is the preferred option, since Apple can verify that there has not been any tampering with the apps since they were signed.
The last option, 'Anywhere,' removes Gatekeeper's blocking capabilities on your Mac, letting you install and open any app off the internet, including Automator apps made by individuals. Sometimes Apps that fall in this category contain malware, We do not recommend using this option and instead recommend using the 'identified (trusted) developers' option above and okay installation of apps missing a Developer ID on a case-by-case basis.
If you have the 'Mac App Store and identified (trusted) developers' option selected, then when you download an app without a Developer ID and try to open it, you'll get the following warning.
To bypass this restriction, select 'OK' on the popup to close it, then go into the 'Security & Privacy' section in your 'System Preferences.' Below the Gatekeeper options in the 'General' tab, you will see the app that was blocked from opening. To trust it, select 'Open Anyway'.
If you do not want to go into Systems Preferences to open an app for the first time as shown above, then Control+Click on the app in the Finder, then select 'Open.' You will see a new popup asking if you're sure you want to run the app on your Mac. Just hit 'Open' again and the App will launch as expected.