The Turn Duct Centerlines On command turns duct centerline layers on. These layers are typically on a non-plotting layer that will not appear on your printed drawings. It is often helpful during the design to turn these centerlines on and off to see what your final drawing will look like.
To turn the centerlines on, go to
Ribbon: DM HVAC->Layers-> 
Turn Duct Centerlines On
Pulldown Menu: DM HVAC->Layer Management->Turn Duct Centerlines On
The centerline layers will be turned on and will be visible on the drawing.