Mac OS X Mountain Lion introduced several new features intended to keep your Mac safe from malware and other security issues. One way they did this was by introducing a program wherein software developers could register with Apple so that Apple could test their software and certify it free of these malicious additions. By default, all Macs are set to refuse to install software that is not from one of these identified developers.
Most Mac developers have taken advantage of this system and registered their software, so programs like Microsoft Office, Flash, etc., install with no problem. However, you may run into a situation where you need to install a program from an unsigned developer, and the good news is that you still can.
First, be sure that the software you are installing is from a trusted source. By installing unidentified software, you do run the risk of putting something on your computer that you do not want. So only do so if you feel confident it is safe and free from malware.
To adjust the security settings, click the Apple symbol at the left of the menu on the top of your screen, then choose SYSTEM PREFERENCES. In the System Preferences panel, click SECURITY & PRIVACY.
If the padlock icon in the lower-left corner of the window is locked, click it and then enter your computer password. You will need to be an administrator on the computer in order to make this change.
Make sure you are in the GENERAL tab of the Security & Privacy preferences. Under ALLOW APPLICATIONS DOWNLOADED FROM, click ANYWHERE.
Now your Mac will allow you to install software from any source. You may want to put this back to MAC APP STORE AND IDENTIFIED DEVELOPERS (the default setting) after installing what you need.
Jason Merrill, Mac & iOS Consultant