WordPress Theme Modification Help

Posting this as a separate answer, because it’s actually different from @Keoki Zee’s answer.

You do need to create a custom menu, but you do not need to use a custom link.

  1. Go to Dashboard -> Appearance -> Menus
  2. If necessary, Create a new menu
  3. Add whatever Pages to the menu that you want
  4. To add a Category, ensure that the appropriate metabox is available:

    4.1. Click Screen Options

    4.2. Ensure that the Categories is enabled (checked)

  5. In the Categories metabox, select the Categories that you want to add to your menu, then click Add To Menu

  6. Save the menu
  7. Add the Menu to the appropriate Theme Location, in the metabox in the upper left-hand corner, titled Theme Locations