There is no official guide yet, but I am writing here how it can be done:
- make sure all the files needed for rendering are in the same directory, with the .lxs file in the directory
- zip the directory content (not the directory itself)
- upload the .zip file in our interface

Alternatively, you can upload the same files on FTP. If you choose to do that, please first create a directory in the FTP root, then upload the files in that directory. After uploading, the files will become visible in our web interface where you add the job.

More details on the FTP uploads here: http://render.uservoice.com/knowledgebase/articles/183344-how-ftp-uploads-work

From sketchup, I mean through LuxRender SU2LXO