Hi,
To use items created in Blender you will need to export in a mesh format thet Terragen can import.
Generally the best export path from Blender is .obj (You will find this on the File drop down menu. Choose file-export-wavefront(.obj)
In Terragen use Add Object .obj reader from the Objects tab.
This is a very simple instruction so that you know where to look for the features. For further detail and workrounds search the forum and you will find plenty of info on shading of imported objects, normal alignment...and a whole heap of other stuff..
Persevere with both programs, I know from personal experience that Blender UI is a little idiosyncratic, but the application is extremely capable.
You may want to explore the possibilities of rendering in separate applications and then compositing them as part of the back end of your workflow.
Hope this helps a bit...
Rgds
Chris