To install it as
root, you will need to mount the image first:
$ hdiutil attach bitnami-mampstack-7.3.13-0-osx-x86_64-installer.dmg
expected CRC32 $65D6DA0E
As shown above, in my case it was mounted to
/Volumes/MAMP. With the image mounted, I can now run the installer with
sudo to make Apache listen on port 80:
$ sudo /Volumes/MAMP/MAMP.app/Contents/MacOS/installbuilder.sh
The graphical installer will open. Follow the steps to install the MAMP stack.
Let me know how it goes.