This should be installed via composer, for now. You can either install it globally, or in your own project. If it is the first time you globally install a dependency then make sure you include ~/.composer/vendor/bin in $PATH as shown here.
It is suggested that you use the phar, as you can get conflicts with dependencies by including it in your project or globally!
$ curl -sS http://bldr.io/installer | php # Or $ composer require bldr-io/bldr "~4.2.0"
And that’s it! From here, you should be able to run bldr if you set it up globally, or ./bin/bldr or php ./vendor/bin/bldr if you set it up in your project.