how do you toggle this on with karma render engine?

Karma is a separate thing in Houdini. If you go to the Stage, that is where you use Karma. There are a couple really great tutorials on how to use Karma. After you put in LOP's you then add Karma render to the end of the stack. If you also go to the drop down tab on the screen where you can change view you can do a Karma render from there. You have to be in Stage or a Stage LOP.