In older versions of InfraWorks you could import 2D AutoCAD objects such as lines, arcs and polylines. In InfraWorks 2015 that ability does not exist. You can import Civil 3D objects such as alignments and surfaces and even 3D AutoCAD objects, but you can’t import 2D AutoCAD lines and polylines.
 
You can, however, use a workaround — the Mapexport command. Using Mapexport means you must use either Map 3D or Civil 3D rather than just AutoCAD.

Mapexport exports an SDF file. You can import the SDF file containing your 2D AutoCAD objects into InfraWorks.
 
I find authoring 2D linework in AutoCAD the easiest way to create some InfraWorks objects, especially if precision is desired.

Notes from Cadalyst tip review R.K. McSwain: We are not sure why the ability to import 2D objects was removed from Infraworks 2015, but this is a good workaround.