The VirtualBox machine image is hosted here: <snip - removed because it's out of date>
You need to unarchive the file, and import it into VirtualBox. The machine will boot into a linux login prompt. Log in as instructed on screen and it will start rendering previews for the lrmdb. The VM requires full internet access to upload/download flm and png files.
If left running, it will iterate over all the materials in the database, gradually improving the quality of each of the previews in turn.
To stop the renderer, just close the machine - don't worry too much about stopping the script or logging out etc.