Reactotron comes in two versions. A desktop version and a CLI.
Download OS X App from GitHub release and drop the Reactotron.app to the Applications folder.
Reactotron is also available via Homebrew Cask package manager. To ensure you get the latest version, update Homebrew first:
brew update
Then simply type:
brew cask install reactotron
If you like to kick it old-school and use the terminal, Reactotron is available via NPM.
To install globally:
npm install -g reactotron-cli
In order to pickup updates going forward, you can type the same command again.