I tried to install Maptool on my Mac, which runs Sierra (Mac OS 10.12.x), and got the "damaged and can't be opened" error message.
The steps in the sticky post Corrupt DMG or Mac App wont install, no longer work under Sierra as the "Allow apps downloaded from… Anywhere" option in the Security and Privacy… General tab in System Preferences has been hidden, and the shortcut to bypass Gatekeeper's warning and open an app doesn't work.
To enable this you have to enter this command in Terminal. You will need to enter your Mac OS X admin password. (Here's the source for the fix.)
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sudo spctl --master-disable
If you want to undo this, enter the same command into Terminal, but change "disable" to "enable":
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sudo spctl --master-enable