You can find the instruction manual for the Juki MO-816 class DD4 Industrial Overlock Machine on the manufacturer's website.
It's a PDF so you can download and print or just view it online. The 15-page document shows you how to thread the machine, install different needles and presser foots and how to maintain or fix the machine if you have a problem.
There are a bunch of videos to watch on the website too that can help you learn more about your Juki machine. Whether you are using it for drapery, upholstery, quilting, canvases or tailoring you can find out more on the site. Have a look and see if it is helpful.
Other useful documents are the standard Juki machine parts books where you can see charts of different machines made by Juki in a lot of detail.