Whenever a new version of OSX comes out, there is generally a bit of wait time before new versions of your software is approved and "signed" (aka certified) by Apple and placed in the App Store. If you want to attempt to install software before this process is complete, you must change a setting in your System Preferences to be allowed to do so:
1) Click Finder then Applications then System Preferences
2) Click Security & Privacy & then click the Lock icon in the bottom left of that window
3) Make sure you have the option "Allow Applications Downloaded from: Anywhere" selected and click OK, then reboot your computer. See the graphic below:
