I have a plane with a grid on it, and when I click it I need to know where the mouse clicked on it in world coordinates. I’m using JavaScript and Unity Free.
http://docs.unity3d.com/Documentation/ScriptReference/Camera.ScreenToWorldPoint.html
(This question has been asked and answered several times before.)
Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousPosition.y, distance));
This will transform the mouse position in screen coordinates to a world position a “distance” away from the camera