Autodesk ARX Enablers
Autodesk ARX Enablers (or Autodesk Object Enablers) are Autodesk freeware applications that enable access to drawing objects out of their native environment.
If a .dwg file is created in (e.g.) AutoCAD Civil 3D it is quite possible that such file contains objects that are unique for AutoCAD Civil 3D. These objects are not visible in "regular" AutoCAD (or some other product that you might have). In order to make them visible in this particular case it is necessary to install „AutoCAD Civil 3D Object Enabler".
What object enabler is required depends on the application that originally created the file you are trying to open. You can ask this the person from which you have received the file.
You should start the search for the appropriate Object Enabler on the page Object Enablers.
There you need to find Autodesk product that YOU are using and follow the link.
On the new page find the program in which the file was original created.
Download that object enabler and install it.