The forum your created has a URL (an active link). You can link to it in a custom menu by creating a custom link to it so there is a forum tab in the horizontal menu on the top of your blog.
A custom menu allows you to display Custom links in tabs along the horizontal navigation where normally only Pages tabs are displayed.
1. You create a custom menu. http://en.support.wordpress.com/menus/
2. You add the Parent pages you want to display and arrange them in the order you want.
3. Then you add and drop and drag the subpages below the appropriate Parent pages, and when you have everything arranged in the menu that you want you save the menu.
4. Next you add your custom link to your forum. http://en.support.wordpress.com/menus/#adding-custom-links
5. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pulldown labeled “primary location.” Click the save button in that module.
5. Refresh your browser so that it isn’t possibly pulling a cached page, and view see your new menu with Parent pages and dropdowns to sub-pages.