In your case, your best option is to simply add a comment to their answer. They will see a little notification when they log in next.
But, for all the details ...
There is no built-in messaging system as it was decided that StackOverflow should be specifically for Q&A, not social networking.
However, you can do one of the following to contact a user:
- Comment on one of their questions
- Comment on one of their answers
- If they have commented somewhere, reply with your own comment, and reference their name like @
- Specifically, the first user in descending order of comment reply time who matches the first three letters of your @name is notified of your comment.