Did you create a folder under "My Documnents/Plan-G Files called DEM? Did you then populate the folder with the appropriate DEM files from http://www.ngdc.noaa.gov/mgg/topo/gltiles.html. After downloading them you need to "unzip" them and put them in the DEM folder. Re-start Plan-G, create a simple flight plan. Then, under the "View" tab in Plan-G make sure the "Plan Elevation" is highlighted.