From LuxRender Wiki
This document is an overview to using LuxBlend 0.1, the interface between LuxRender and Blender 2.4x. LuxRender is a rendering program, something that takes a scene created with a 3D modeler and renders it (as LuxRender has no modelling tools of its own). By installing LuxBlend you connect Blender to LuxRender and you can use Lux to render your scenes instead of the Blender internal render. Instructions for installing LuxBlend can be found at Installing LuxRender and LuxBlend.
Starting LuxBlend within Blender
The general workflow for using Blender/LuxBlend/LuxRender is indicated below.
First the bad news: Materials created in Blender do not translate over to LuxRender, as the material engines in each speak different languages. Therefore, while LuxBlend can attempt to 'translate' the materials in a Blender scene file, it is advised that you check all of the materials in LuxBlend or set them up manually before hitting its 'Render' button. Additionally, LuxBlend cannot assign a material to an object that doesn't have one, so you need to make sure a material is assigned to each of your objects in Blender before invoking LuxBlend.
With this in mind, the best workflow for creating scenes in Blender is to model all of your objects (including lights and camera), place them where you want them, and then assign a default material to each object. You will want to give this material a unique name, but you do not need to make any changes beyond that as all material properties are set in the LuxBlend exporter. If you intend to use texture maps with UV coordinates, the model will have to be UV unwrapped in Blender prior to setting up the materials.
Once that is complete, launch the LuxBlend exporter. From this point on, selecting any object in the Blender scene will also select its associated material in LuxBlend. Once you've set up your material and lighting options using the material tab, you can move over to the camera and environment tab to set up the parameters for both your camera (focal length, depth of field, motion blur, etc.....) and the world surrounding your scene (environmental lighting, sun parameters, etc....). Then you can move into render settings and define how the image will be created (integrator, pixel sampler, volume integrator, etc...). Finally, you should verify that the output infomation is correct. After that it's just a matter of clicking 'render' and watching your image appear.