After some experimentation with my own forums, I tried this out:
1) Post your poll (set it up).
2) Find out the forum ID for the topic/forum - I couldn't find that out, and as my forum has one poll, I left the space blank to get the poll from all forums.
3) Go to Administration Control Panel, and "Portal". Then "Poll" and Display poll blocks - put yes.
4) Experiment a bit. I have no idea whether this forum is PHBB3 or not - mine is.
I hope that helps!