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Login to unpack the .7z file.
Go root, via "su" or "sudo bash" or whatever your favorite method is.
Determine what device your SD card is on. On my Macbook, it was /dev/disk2. If it is mounted, you can find this easily by typing "mount". Be sure to unmount it before running dd. You can unmount via "Disk Utility" or "diskutil unmount ...".
Run "dd if=MedievalMadness2Installer.img of=/dev/disk2".
Take a nap, lunch-break, or walk away for a few hours. This takes a long time. Make sure your Mac isn't setup to fall asleep while doing this.
I think you may be able to do the equivalent (after unpacking) with Disk Utility, but I prefer dd.