This message is a security setting on Mac computers.
To bypass that message, hold down the "control" key on your keyboard while clicking on the installer file. Then click "Open" on the top of the menu. A new message will pop up with the option to "Open." Click "Open" to allow it to run on your Mac.
That should bypass that message!
You can also change your security settings by going to System Preferences on your computer, then to Security & Privacy.
Under the General tab, make sure "anywhere" is checked underneath "Allow applications downloaded from."
Then re-try. That will also bypass that message!