Do you see a button at the bottom of the post labelled Actions ? (Make sure you are signed in as Jive the host has a habit of NOT remembering your credentials)
If you click that it has a list of things you can do including mark as helpful.
Thank you for posting your inquiry in the Help forum. Authors of a discussion can mark a reply as Correct Answer, but not Helpful. I encourage you to use this option if you identify a correct answer and you are the author of the post.