Procedural terrains are great at simulating hills and mountains, but some scenes just need objects – trees, houses, cars, or pretty much anything else.
You can create your own objects using 3D modeling tools like Maya, Lightwave, SketchUp and many others. That requires specialized skills.
There are tons of models already out there. Some can be purchased, while others are completely free. A good starting point for free models is the Google 3D Warehouse.
There are many format out there for 3D models, only some of which can be imported into Terragen 2. Files in the .obj format and in the .lwo format can be imported directly. The popular .3DS format can be converted to one of those two formats using the free Anim8or program.
Once you have your object (and possibly an associated texture file that tells Terragen 2 how to render the surface… in the case of .obj files the associated file is a .mtl file), you’re ready to place it in your scene. This video shows you how (there’s no sound, view full screen and select the best resolution you can):
A sample project complete with Ducky (d2.obj) is available here.