Under Forum Administration > Manage Forums/Groups, go to the required forum and set the Forum Permissions you require by checking the various check boxes as required:

1. View: Select all roles that can view this forum. - ensure All Users is selected for this one
2. Start Thread: Select all roles that can begin a new thread in this forum.
3. Post Reply: Select all roles that can reply to a post in this forum.
4. Moderator: Select all roles that can moderate on this forum.
5. Add Attachments: Select all roles that can add an attachment to their posts for this forum.
6. Pin Thread: Select all roles that can pin a thread on this forum. Pinned threads will appear at above other threads.
7. Lock Thread: Select all roles that can lock a thread on this forum. Replies cannot be made to locked threads.
8. Notifications: Select all roles that to receive moderator notification from this forum.
9. Unmoderated: Select all roles that do not require moderation.

Note that some options will be disabled depending on the Forum Type you have chosen.

Dear Matthias,

First of all thank you for your answer, I'll let you know if it worked as soon as I can try it.

I'm sorry about the double posting. I didn't mean to post it twice it's just that I clicked back button of my browser to make a correction on the post and it created a new thread instead of changing the original.

I apologize for any inconvenience I may cause.

Best regards,

Xavi