Adding buttons is as simple as a drag and drop but first you need to get into the pos screen layout options.

If you go to Setup > Pos Screen > Pos Screen Setup you can then select the layout you are using, you can create as many layouts in here as needed which helps to build the layout for a new menu or different stores

Once into the editing screen of the layout you will notice a listing of all your stock items on the top right of the screen, you can search directly for an item or find it in the listing. Once you have found the item drag it down onto the pos screen to create the button for it.