Why don't I see an answer button?
There will not be an answer button on questions that have closed. Each question has a 4-day open period during which people can post answers. A question is closed to additional answers when a best answer is selected or when it's put to a community vote. Once a best answer is determined, an Open Question changes status and becomes a Resolved Question.
Even after a question is resolved you can continue to contribute to Answers by commenting on and rating the best answer. Ratings become available once you reach Level 2.