The cause to this issue might be related to the fact that you're using a wrong version. The official website offers for downloading version from 2013 up to 2015. Access the page, then select the version of AutoCAD, architecture type and language. Secondly, the plug-in is available for the vanilla version of AutoCAD without Civil 3D installed. This could be a serious issue with the Civil 3D installed. Check the version of your AutoCAD then download the plug-in.