So, you downloaded Mountain Lion, and everything was good. Then went and grabbed a new third-party update for a program only to find that when you double-click it you are greeted with a new dialogue box telling you that you can’t do that. What the heck?
Don’t fret, it is supposed to work that way and unbeknownst to you, it’s in your settings to behave that way as the default, but you can change it easily enough.
The default is to allow only Mac App Store apps to install by double-clicking, which is a change from the old way where double clicking gave you the option to install. Now if you want to install you need to right-click or control-click the app and select “Open” and you will be greeted with the second screen grab.
It may seem a tad goofy at first, but it’s all about security and meant to work that way to help stop you from installing something that could be dangerous. Viruses, for example. Yes they do happen on Macs just not very often… yet. Those nefarious types are trying though. So far Windows is an easier larger target to hit, but some do like the challenge of the Mac OS and are playing around with little nasties to infect our pretty little Macs.
Getting back to the settings, if you go into System Preferences, in the tab for Security and Privacy (shown below) you can change the setting, but it will make you more susceptible to any scams and viruses that may be disguised as apps you wanted to download and install.
But as the warning says…
My personal preference is to leave the setting as is and be more secure. Just have to retrain myself to not try to double-click apps to install them. That adage about old dogs and new tricks comes to mind, but it is better to have to click a couple more times to install something, than to mistakenly install something nasty that could take hours to remove properly and irrevocably damage or delete irreplaceable files. Don’t treat installers like the way most treat EULAs. Make sure you know what you are installing and meant to do it.