First tap the ethereum brew:
brew tap ethereum/ethereum
For the latest stable version:
brew install ethereum
brew linkapps
For the latest cutting edge developer version:
brew reinstall ethereum --devel
brew linkapps
Then open AlethZero.app from your Applications folder. You can also run eth (CLI) or neth (ncurses interface).
For options and patches, see: https://github.com/ethereum/homebrew-ethereum
If you're just interested in the GUI client AlethZero without the command line interfaces, download the latest stable or cutting edge version.