Getting blocked by OS X’s Gatekeeper with a message that it “can’t be opened because it is from an unidentified developer”? You could turn Gatekeeper off, but it’s there for your safety. Instead, if you are sure that a program is from a trusted source there is a way to bypass it temporarily. Instead of double-clicking the app or the installer, right-click it (or two-finger click or control-click) and choose “Open” from the popup menu. This presents the warning but also gives you a button to let you install it anyway.
What is Gatekeeper? It’s an anti-malware feature introduced in OS X Lion that aims to make it more difficult to install software with unintended consequences (malware or trojans). Any registered developer can sign their software and distribute it, so it’s not like the App Store where it has to be pre-approved by Apple. If the developer is found to be making malicious software then Apple can flip a switch so that Gatekeeper blocks it (though that system has not yet been used to my knowledge). Gatekeeper also blocks software that is determined to have been modified by someone other than the developer (possibly the most common source of trojans on Mac) and software that hasn’t been signed at all (common with older programs and open-source software).
If you are an existing customer who needs help with this or if you have other questions, or if you are in San Francisco and interested in becoming a client I invite you to book an appointment with me. Otherwise, you may wish to contact Apple Support or to find a local Apple consultant.