how do I enable the root user from the Directory Utility?
You don't need to put anything in there if you're working locally, just click 'Open Directory Utility' [which you did, based on your second pic]

Then…
Edit menu - Enable root user [only available once you unlock Directory Utility, which it looks like you did]

...I would wonder why you need root unlocked - you really don't need it in OS X, you can sudo anyway