Did you try looking at the account settings rather than the editor? I'm not sure about being able to do it from the editor at all. I don't use the editor, old or new, to post to LJ so I don't pay a lot of attention to what it can do. I'm sure about "enable comments" being in the account settings for me though. That's not the kind of thing I'd expect to depend on which theme you were using so I think it ought to be there for you too.