If your theme supports custom menus as I think that is what you did then you should be able add more to the menu
It does say, “Your theme supports 1 menu, which menu would you like to choose?” I know that kind of answers my question, that you can only have one extra menu, but that just doesn’t seem to make sense. There has to be another way to add another menu button. Why would you only be allowed to add one extra menu button? Is there something I can do with the coding? Is there another way to add a menu button, instead of the way I added the extra menu button?
under that box you should have pages box and category box and post box if you do not see them click on screen options on right hand top and tick the ones you want and then it matter of adding them by ticking and then click add to menu then in menu arrange them by dragging as you wish and save the menu