That is truly fantastic. I love the setup, but I'm most impressed with the colouration and texture of the forground. I can never seem to get grass and rocks to look like that! Looking forward to further updates on this one!
As far as object placing is concerned I just tend to move my mouse over the preview pane to the rough area that I want and then type those values into the translate coordinate boxes for the object. Then I move the camera around and click and drag the object around the preview pane for fine adjustments. Takes a while, but usually works. I also do very rough sample renders to check the positioning (about 320x240, detail 5) before repositioning as required.