When attempting to download and install a program to a Mac OSX 10.9 to 10.11 operating system, you receive an error the installer can't be opened because the identity of the developer is unknown or you receive an error that the file is damaged. 

To correct this, simply disable the security setting prior to installing the program. Then when finished installing, the security settings can be re-enabled.  

To temporarily disable your Mac's security:

  1. Open System Preferences from the Apple Menu. 
  2. Click Security & Privacy. 
  3. The General option will be highlighted, if it is not click on it to highlight it. 
  4. Click the padlock in the lower left corner to allow changes and enter an administrator password if prompted. 
  5. Under "Allow applications downloaded from" select the "Anywhere" option. 
  6. Click the "Allow From Anywhere" button in the dialog that pops up. 
  7. To re-enable security simply switch the "Allow applications downloaded from" setting back to "Mac App Store" 

When receiving the above errors on Mac OSX 10.12 operating system the Anywhere option may not be available in the Security & Privacy settings.
The steps to add the Anywhere function can be found at the following location: http://osxdaily.com/2016/09/27/allow-apps-from-anywhere-macos-gatekeeper/
If you are not comfortable making these changes to the computer, please contact Apple for further assistance in setting the Anywhere function on the computer.