The Menu is driven by the pages so if you want a menu item for Forums create a new page and name it Forums. Then in Content Manager you can Publish the instance of forums on the page.
Note that you can also create feature instances without ever going in to Content Manager. Click the pencil icon on the page you are on to add new feature instances to the page. So you could have just created a new page and then added a new forum to it, but now that you have a forum instance created you might as well just use it, so publish it on the page from Content Manager.
Click around on the icons and I think you'll be up to speed soon.
Glad to have you using mojoPortal!
Cheers,
Joe