There is now the option to install directly using homebrew. First install MacTeX, either directly from its website, or using homebrew,
brew tap caskroom/cask
brew cask install mactex
Then install cadabra using
brew tap kpeeters/repo
brew install cadabra2
These instructions are now also on the download page. Please let me know if you run into any trouble; this has not yet received a lot of testing!