If you look towards the top, below the Forum/What's New? tabs, you'll see a number of options. The one you want is Forum Actions. A suboption of that is General Settings, which is the one you want. When you go into General Settings, look down the page until you get to Miscellaneous Options. Message Editor Interface is the option you want, and you need to set it to Enhanced Interface - Full WYSIWYG Editing. Then go to the bottom and Save Changes.