Dinesh,
Unfortunately, you can't force the new comment box to be displayed. It is automatically displayed if the number of comments is less than the number of comments you have set to show. Once it goes past that number, a user has to click the Reply or Add Comment link. I believe the idea is to get more responses to existing comments and that users will click the Add Comment link after enough comments are already there (people want to get on board).
Honestly though, I can't speak for exactly what Facebook's intents are, just what they allow us to do... and forcing that box isn't one of them.
Hope that explains,
Alex