If you have installed Apple’s new operating system (Mountain Lion), you will notice that you won’t be able to install software which hasn’t been signed by an identified developer or downloaded via the Mac App Store (this is also known as Gatekeeper). They do this to counter the growing amount of malware that is bugging OSX the last couple of months. As the option is turned on by default, you have to find where to shut it off to download unsigned content (especially older apps).
Go to System Preferences < Security & Privacy < Tab ‘General’
You will see ‘Allow applications downloaded from’ greyed out, so first authenticate by clicking on the little padlock at the bottom left of the window.
Select ‘Anywhere’ instead of ‘Mac App Store and identified developers’
You’re done 🙂