If you've just imported the Dialogue System into your project and the player in DemoScene1 falls through the floor:
Try importing the Dialogue System into a new, empty project. Test the demo scene there.
If it works in the new, empty project, then your project has probably overridden the default Unity physics settings so that collisions with the Default layer don't register. You can resolve this in one of two ways:
1. Select menu item Edit > Project Settings. In the Physics section's collision matrix, tick the checkboxes for Default.
2. Or, in DemoScene1, change the "Room > Plane" GameObject's Layer and Player GameObject's Layer to layers that will register collisions.