It's part of the Business section of TT/Calif. Go to the Business and Edit it. One of the first screens after that (it was the first screen just now for me) is Is <name of biz> owned by a single member LLC? Clicking the Yes button activates a 568 LLC return associated with that Schedule C.
A Rental property(ies) LLC is activated differently but I'm assuming you're referring to a Schedule C business.
