Click on the apple in the top left hand corner, go to System Preferences > Security & Privacy > General and change the following setting:
Under the section labelled Allow applications downloaded from, select Anywhere. On the prompt that appears, click Allow From Anywhere. Be sure to revert this setting after the install is complete.
OR
When opening the downloaded launch.dmg file right click instead and click open. When the error comes up you will have an option to ‘open anyway’.