Need Some Help regarding insert new objects in island demo

This is Aamir Malik. I am newbie. my problem is... i open the Island Demo and i just insert the Cube in scene. when i view game mode ... cube is not there. how can I add new cube or model in island demo?

There could be a number of reasons for this. Maybe gravity is causing the cube to fall through the terrain.

Drag the 'game' dock window next to the 'editor' dock window, and then hit play. Watch what happens to the cube in the editor window, that should help you work out what is going on.