Maybe you are looking at the back side of the plane? Try setCulling(false) on the plane to see, if that helps. If it does, consider to rotate the plane by 180 degress around the x-axis. If it doesn't, make sure that you don't look at the plane from the side, because it has no thickness.
BTW: Your plane is really huge. Is that intentional? It's 20*30=600 units in size in both directions.