Creating and activating the menu
If this is the first time you install a theme for your blog, you will need to create a new menu : navigate to Appearance –> Menus –> type the name on Menu Name box and click Create Menu
Then, check the box “Primary Menu” to assign the current menu as the menu bar at the Top.
Adding the items
Just simple navigate to the tabs such as Pages, Posts, Categories… to add the items to the menu
To create a dropdown menu for categories, you add the items from Categories tab to the menu, then drag and put them like this:
Note: You have to create the categories (at Posts –> Categories) before to add them to the menu.
Be sure you save changes when you finished.