We currently have experimental support for HHVM installations on Mac OS X 10.10 and greater. We use an official Homebrew formula. While you can build from source, it is generally advisable for ease of installation and stability to use the formula.
brew tap hhvm/hhvm brew install hhvm
And then wait a long time for it to compile since we don't provide bottles yet. (This will be anywhere from twenty minutes on a beefy Mac Pro to a couple of hours on a MacBook Air.)