AC3D needs an stand-alone rendering program to render realistic images. Many people here seem to use POV-Ray (with or without PoseRay) or Bryce. In addition to POV-Ray, I also use Poser's Firefly renderer myself.
Here's a list of free, stand-alone renderers that might be helpful:
http://www.independentdeveloper.com/...rendering_soft
EDIT: I believe AC3D comes pre-configured for POV-Ray.