You can use a tool catalog item in Autodesk Civil 3D by inserting a tool directly into the drawing area from the Content Browser. For example, with the Basic Subassemblies catalog open, move your cursor over the image for to the BasicBarrier subassembly to display a context menu. Click Insert into Drawing to insert it into the drawing.
You can move your cursor over an image for a subassembly to display a context menu and then click Add to Tool Palette to add it to a tool palette.