I have this on both an iMac and a Macbook Pro, both running OS-X El-Capitan. When initially running FreeCol, after downloading version 11.6, the message received "To open "FreeCol.app" you need to install the legacy Java SE 6 runtime. After installing Java SE 6, the next message received is : "FreeCol.app" is damaged and can't be opened.".
This same message is received if Jave 8.66 is installed.
Any helkp would be apreciated.
There is no developer with a Mac, all I can say is that there is something wrong with your install or the Mac install procedure, as other people had problems with that, too.
The message about Java 6 is wrong. You need Java 8 and you probably have more luck with the zip or jar download.
That message is coming from your Mac, not from FreeCol. You need to sort out what your machine is doing with Java, which is not something we can help you with.