Chris Perry and I got a case where a user could not run NWCOUT at the command line. That was the case where the ARX downloaded from http://usa.autodesk.com/adsk/servlet/pc/item?siteID=123112&id=13947462 was not loaded into his AutoCAD 2010 MEP even after installation. We are not sure why this happened but we were able to duplicate it on a training machine. Using the search on Chris’s machine and a training machine we found out the nwexport2010.arx file was in different folder locations on each machine!
So the procedure recommended would to run a search for the location of the nwexport2010.arx file. Open the AutoCAD seat and type ARX then browse to the folder containing the nwexport2010.arx file to load it.