Introduction
The exporter plug-in for Blender 2.5x is called LuxBlend 2.5. As Blender 2.5x finally was released as stable in the form of 2.57, LuxBlend 2.5 is now targeting the latest stable release.
LuxBlend 2.5 exposes virtually all LuxRender features and offers tight Blender integration via the binary pylux module.
Installation
The installers for OS X and Windows includes both LuxBlend 2.5 and the pylux module and is the easiest method of installing LuxBlend 2.5. For instructions on how to manually install the addon, please see the LuxBlend 2.5 installation instructions wiki page.
You can also get support in the LuxBlend forum.
Note for Windows users
The pylux module may require the Visual C++ Redistributable Package to be installed, which you can get here (for 32bit Blender) or here (for 64bit Blender). If LuxBlend 2.5 fails to load pylux, try installing the appropriate package.