Do you mean where do you create one or how do you format it?
The buttons for comparative and poll discussions are to the right of the "Start a discussion" button
For a poll:
The "Poll question" textbox is the title, or what appears on the main forum pages. The "Post" textbox is additional information, your views, a further explanation, etc. The checkbox-sections are pretty straightforward I think. And there's a button to add more answer choices if you need more than 3.
For a comparative discussion:
The first page is the normal two boxes--title first, main explanation second. When you go to the next page, you select the products you want to compare (if any). Third page I believe is just a confirmation of the products you want to include. On the fourth page, you can choose to add discussion topics or polls, both of which work as you normally fill them out. Fifth page shows you who you would like to be able to participate in the polls/discussions in the thread.