How to install Luxrender? I need a manual download link for the Blender addon so that I can copy this .py to the add on folder and then I hope I can switch to the Luxrender option in Blender 2.75.

After following the normal installation process there is no addon installed. I am using Windows 7 64-bit operating system.

Any idea what's wrong?

The installation process by Inseln looks good.
Have you tried to follow the installation instruction here?

Below the main instructions there is a link to a development builds repository where you can get a .zip file which also contains Python files. But you have to follow instructions for that package, too.