After installing Civil 3D 2010 make sure to read the ReadMe file. There is some documented bugs and/or pitfalls that you can avoid. One of them is plotting to a DWF when a Pipe Network is data shortcutted into the drawing. The ReadMe file states:
“Pipe networks referenced into a drawing using data shortcuts may not appear in a DWF. This can occur if the drawing is not saved before creating the DWF. To avoid this situation, save the drawing before creating the DWF.”
So read the ReadMe and you may save yourself some time trying to figure out why something doesn’t work.