It's so simple that it took me a while to figure it out too! You go to the Display Menu, at the bottom and find "Guides" make sure "Ground Plane" is checked and then on to Materials in the Render menu. Here you just load your texture and voila! It was always there but unless you had the floor guide checked you couldn't see it. What I want to know is how I can render and still have the ground shadows show. Once I render those shadows disappear. What am I doing wrong NOW?