You need to use something like Nvidia's Melody or ATI's Normal Mapper utility to make normal maps with AC3D. Melody uses OBJ files if I recall. Normal Mappper uses it's own format, for which there's an AC3D exporter. I use ATI's utility myself. (The maps produced are useable regardless of what kind of video card you have.)
Basically, you build two versions of your model... the low level-of-detail model which is the geometry you'll use in-game, and the high level-of-detail model you want to extract the normals from. You export both to the normal mapper format, then run the normal mapper utility against the models. The normal mapper utility will raytrace the normals from the high LOD model and project them into the texture space of the low LOD model. The low LOD model will need to have texture coordinates applied before you start. The high LOD model does not. The normal mapper utility has more detailed instructions on how the models need to be set up.
This will generate the map. You'll need another utility to preview the results. I generally connect one of the GLSL shader utilities to the AC3D "render" button myself.
If you don't have the plugin, you can download it here:
http://www.independentdeveloper.com/...l-map-exporter