Firstly, make sure you download DrawingPanel.java from the Program 3G folder on moodle. Upon downloading the DrawingPanel.java file, you should drag it into the src folder of your "Program 3G" in Eclipse. This should be all you have to do in terms of file placement.
If you have done this and your program still doesn't run, you may have forgotten to include "import java.awt.*;" above "public class ChristmasTrees". Hopefully this helps.