When you start a new thread you'll see below the text box a clickable link that says "poll manager" - click on that, then it will ask you if you want to add a poll, then just follow the steps from there. When you get to the end it will say "add poll" or "done" - click on "done adding polls" or else you'll wind up adding a whole other poll to your thread.
Be warned, when you preview your post, the poll does not show up. It only shows up after you submit the post.