These are instructions on how to run applications that 'can't be opened because it is from an unidentified developer' in Mac OS X Mountain Lion.
ALLOW SINGLE APP:
If you would like to allow just one specific application to run:
Step One
Hold down the Control key and click the application icon. From the contextual menu choose Open.
Step Two
A popup will appear asking you to confirm this action. Click the Open button.
ALLOW ALL APPS:
If you would like to allow all applications from unidentified developers to run:
Step One
Select System Preferences from the Apple Menu at the top left of your screen.
Step Two
Choose Security & Privacy from the System Preferences menu.
Step Three
Select General from the tabs at the top of the Security & Privacy page.
Step Four
Click the Lock icon at the bottom left of the window.
Step Five
Enter your administrative username and password and click Unlock
Step Six
Select Anywhere from list of places to allow downloaded applications from.
Step Seven
A popup window will appear asking you to confirm your selection. Click the Allow From Anywhere button.
ALLOW SINGLE APP:
If you would like to allow just one specific application to run:
Step One
Hold down the Control key and click the application icon. From the contextual menu choose Open.
Step Two
A popup will appear asking you to confirm this action. Click the Open button.
ALLOW ALL APPS:
If you would like to allow all applications from unidentified developers to run:
Step One
Select System Preferences from the Apple Menu at the top left of your screen.
Step Two
Choose Security & Privacy from the System Preferences menu.
Step Three
Select General from the tabs at the top of the Security & Privacy page.
Step Four
Click the Lock icon at the bottom left of the window.
Step Five
Enter your administrative username and password and click Unlock
Step Six
Select Anywhere from list of places to allow downloaded applications from.
Step Seven
A popup window will appear asking you to confirm your selection. Click the Allow From Anywhere button.