How can I make a dialog show up in game?

There are two ways of doing this; using gdscript or the scene editor.

Using the Dialogic class you can add dialogs from code easily:

var new_dialog = Dialogic.start('Your Timeline Name Here')
add_child(new_dialog)

And using the editor, you can drag and drop the scene located at /addons/dialogic/Dialog.tscn and set the current timeline via the inspector.