If the administrator has enabled this function on the board, you'll see a 'Report' button in a post, next to the 'Quote' button. This function will let you report the post to the forum moderator (or the administrator(s), if there isn't a specific moderator available). You can use this function when you think the moderator(s) should be aware of the existance of that post. However, do not use this to chat with the moderator(s)!. You can use the email function or the Personal Messenger function for that.