Look up! (to the top of the topic) Up at the top in the "Important: Read before posting" there are instructions.
The basics are, add a reply to the topic - up at the top click the "manage this topic's poll" button, and put Solved in the question text box. You can add more info in the answer box if you want (like which post has credits, or who gave the correct answer).
& don't worry about it not working completely the way you want it to the first time! (or two, or three times ... ) It takes some getting used to!