Each button has to have an action assigned. The plugin supports 6 actions currently, with more to be added in the future.
Scroll to the top has one option: time needed for the scroll to reach the top of the page. For custom URL you need to specify URL for the button to go to, and for website home page you have no additional settings.
Popup with Menu
If you select popup with menu action, you need to select Menu designed with WordPress Appearances -> Menus. All available menus will be listed. No matter how the menu looks, the plugin will use only first level menu items and it will apply some basic styling.
The popup can be styled very similar to the buttons. This means that you can set background and text color, size for the popup (height can be 0 for auto height), radius, padding, and offset from the Swift button. Extra CSS class can be specified too if you need to apply more styling to the popup.