Your+security+preferences+allow+installation+of+only+apps+from+the+Mac+App+Store+and+identified+developers

By default Apple has changed the security settings to only allow installation of apps from the Mac App Store and identified developers. To change this permanently or temporarily go to System Preferences > Security & Privacy. On the General Tab click the little lock in the lower left corner to unlock the general preference pane. Then select the Anywhere radio button beneath Allow applications downloaded from: Here are a couple screenshots.

