Grid generation uses raycasting to find the ground positions for cells. Make sure the component's raycast settings are correct, e.g. only using layers of the ground and having a large enough distance from the grid's game object to the ground to find it.
You can also generally follow the ORK 3 tutorials for the 2D grid battles - the setup is basically the same for 2D and 3D, just that 2D changes the default horizontal plane :)