For reference (and others reading this post), instructions for installing the Juicebox plugin for Lightroom can be found here.
It sounds like your 'juicebox.lrwebengine' folder may be in an incorrect location. (You are certainly looking in the right place in the Lightroom interface. Once installed, 'Juicebox' should be displayed as an option in the 'Web -> Layout Style' section.)
As you are using a Mac, you should be able to just double-click the 'juicebox.lrwebengine' package to install the plugin (rather than manually copying it to the correct folder). Please note that this will not work for Windows users.
There are also instructions for installing Web Engines (such as the Juicebox plugin for Lightroom) here (with instructions on how to access hidden folders if necessary to find the correct path).
Hopefully the information above will help.