Have you looked at the demo geds? Also there are plenty of tutorials on the forums, and located here.
http://game-editor.com/TutorialsTo add a floor, you need to create an actor, and give it an animation. Easiest way to create levels is by making this actor a tile actor. After that you need the player to have a collision event with the floor, and use PhysicalResponse. I'd suggest going through the demos as they teach you these basics.