Ribbon Home tab > Palettes panel > Tool Palettes button
From the Tool Palettes dialog select the desired tab for catagory of sub-assemblies, then select the appropriate subassembly.
HINT: To better understand a subassembly right-click it in the Tool Palette and select Help.
In the Properties dialog, under the Design tab, you can adjust the parameters of the subassembly to match your project.
Once the parameters are correct select the assembly baseline marker or another subassembly marker to insert the subassembly.
NOTE: For ease of creation the subassemblies can be created on either the right or left side of the assembly baseline and then mirrored across to form the final assembly.
To mirror the subassembly across the assembly baseline simply select all of the subassembly > right-click > select Mirror.
NOTE: The parameters of the mirrored subassemblies are not dynamically linked. If you change a parameter value for a subassembly on one side of the assembly baseline, the change will not be applied to the opposite side.
NOTE: Use widening subassemblies to stretch design sections.
NOTE: Use Daylighting slopes to tie roadway design into existing ground.
AutoCAD Civil 3D 2012 User's Guide reference: User's Guide - Corridor Modeling - Assemblies and Subassemblies - Creating Subassemblies