Ashok-

I believe you might be running into the same sort of problem I was having earlier today. The Forum module documentation isn't totally clear on this.

You first need to ensure that you have a Security Role created (for example, Forum Users) that has Edit rights to the particular instance of the forum module. For ease of populating the group, you may want to make it an Auto Assignment role so that all existing registered users get placed in the role. Then, go into the specific forum, and make sure the same Security Role is listed in the Authorized Edit Roles box (if necessary, search for and add the Role here). At that point, if you are logged in as a registered user, you should have the New Thread option.

Non-registered viewers will still be able to view the forum, but will not have New Thread available, or even displayed to them. I'm using this module to set up an intranet BBS system, and while I see its power, the lack of beginner documentation has led to some stumbling blocks. I hope this helps...

Mike