By default forums are moderated. This means that a site manager must approve a post before it appears in the forum. You can go to Content management->Moderated Content http://yoursite.com/admin/content/modr8 or for easy access to which posts need to be moderated you will want to enable the Moderation queue block.
The Moderation queue block will show you how many posts are in the moderation queue. Clicking on it will take you to the moderation queue.
The posts will appear or will be deleted as specified.