Some users leave means of contacting (email, Twitter, personal website) on their profile. If not, a comment is about the only option within the current (and, likely, future) Stack Exchange system. It's not exactly what they're intended for, but one could argue that you're "requesting clarification from the author". In any case, as soon as they respond, be a good citizen and clean up after yourself by deleting the comment(s).
(I do not have enough reputation points to post it as a comment.)
That's too bad, but it's rather easy to earn 50 reputation to obtain this privilege. It's less than the association bonus, so if you do have experience about an entirely different topic which is covered on another Stack Exchange site, you'll be able to comment everywhere in the network.