Get 50 reputation (writing a couple of good answers, suggesting a load of edits, asking some really interesting questions, etc) and then you can comment. This barrier —annoying as it is— is to help prevent spam as well as other things that shouldn't be comments that are more likely to come from brand new users.
If they were active on Chat, that might be another route but you also need at least 20 rep for that.
But they're not the only ways to find somebody.
They have a profile on this site. Click the avatar. They might have contact information listed. It might seem weird but if it's in there, it's fair game. That's them saying how they want to be contacted.
Is their username unique? Can you find instances of that elsewhere on the internet?
You could even crank up the creep factor to 11 and throw their avatar through Google Images to find them in other places.
Hack PRISM (the US surveillance program), find logs of traffic that could have only been from them (eg the post URL for that question), hack their ISP to get their billing info and give them a phone call.
But ideally, this is exactly the sort of question that's best for a comment.
Earning 50 rep is fairly easy and involves less prison time than some of these options.