Hi:
You haven't missed anything -- I'm afraid there isn't a way (at least that I know of!) to mark a particular answer as helpful, or mark a question as answered. I always really appreciate it when I see people add a post to their thread mentioning that they have solved their problem, and adding any additional details about the final solution. That way other users can see what may have had to be done differently, or what the outcome was.
There is a star rating system for a thread overall -- you could mark the thread with more stars if your problem was solved -- that seems like a good use of that feature.