LuxRender is one of the two render engines supported by the Raytracing Workbench together with POV-Ray. In 2013 the project has been rebooted becoming LuxCoreRender, with a major code rewriting and compatibility breaking changes, thus the LuxRender project has been abandoned. Officially LuxCoreRender is not supported by the Raytracing Workbench, although it might be worth to try.
The latest stable version is LuxRender 1.6 (2017-12-28).
As LuxRender is no longer developed, almost no distribution still has it in its repositories, so you will have to manually install the official binaries. First determine if your graphic cards supports OpenCL and if you have all the necessary dependencies installed. If so, download LuxRender 1.6 with OpenCL support. Otherwise download LuxRender 1.6 without OpenCL, but consider getting a newer graphic card or installing the necessary software to enable it on your computer.
The faster solution (although not the best practice) is to extract the content of the archive in a suitable location, like ~\LuxRender. If needed change the file permissions so that your user can execute the files you just extracted.
Compiling from source
If your distributions does not have LuxRender in the repositories and the official binaries do not work on your distribution, or if you wish to, it is possible to compile LuxRender from source. Download the source code of LuxRender 1.6 from GitHub
Set the Luxrender executable path to point to your installation of LuxRender, in particular to the luxrender executable, and apply. In the above example, the path would be ~\LuxRender\luxrender.
First determine if your graphic cards supports OpenCL and if you have the necessary graphics drivers installed. If so, download LuxRender 1.6 with OpenCL support setup. Otherwise download LuxRender 1.6 without OpenCL setup, but consider getting a newer graphic card or installing the necessary software to enable it on your computer. Then execute the downloaded installer and follow the proposed steps.
By default the destination folder is C:\Program Files\LuxRender, with example scenes in C:\Users\Public\Documents\LuxRender\Example Scene.
Set the Luxrender executable path to point to your installation of LuxRender, usually it is C:/Program Files/LuxRender/luxrender.exe, and apply.
There is no development version of LuxRender, as development has been halted in favor of the reboot LuxCoreRender.