I would like to see that left side panel in action, but it does not seem to be an option on this site? Is it disabled or is there something else I need to do to bring it up?

On this site you can't enable it because this is in the theme editor which usually is not accesible for regular users.

You could create a trial perhaps?

https://invisioncommunity.com/clientarea/free-trial/

Oh, I thought the sidebar was just another view option the user selected.

 

Oh, I thought the sidebar was just another view option the user selected.

It isnt, however you could create multiple themes on your own site, if you wanted to provide that option